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The new Karl G. Jansky Very Large Array (VLA) correlator is extremely powerful in its spectral capabilities. The final specifications include up to 4 million channels that can be distributed in up to 64 subbands with a spectral resolution from from 2MHz down to the the Hz regime.   Here is a guide to access that spectral line power and we describe what is offered for the August 2012 deadline.  
 
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The new Karl G. Jansky Very Large Array (VLA) has extremely powerful and versatile spectral capabilities. The final specifications include up to 4 million channels that can be distributed in up to 64 subbands with a spectral resolution from 2MHz down to the the Hz regime. This guide is intended to clarify the spectral line capabilities of the VLA, with a focus on capabilities offered for the August 2012 VLA proposal deadline, and enable users to plan, prepare, and process spectral observations.


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[http://www.vla.nrao.edu/astro/guides/sline/current/ VLA spectral line guide]
[http://www.vla.nrao.edu/astro/guides/sline/current/ VLA spectral line guide]


    Contents
Contents
    INTRODUCTION
INTRODUCTION
    SYSTEM SPECIFICATIONS
SYSTEM SPECIFICATIONS
        Receivers and IF System
Receivers and IF System
        The Local Oscillator Chain
The Local Oscillator Chain
        The Correlator
The Correlator
            Total Number of Channels  
Total Number of Channels
        Some Advanced Spectral Line Topics
Some Advanced Spectral Line Topics
            Creative Use Of The Spectral Line System
Creative Use Of The Spectral Line System
            High Accuracy Spectral Line Polarization Observations
High Accuracy Spectral Line Polarization Observations
            The Lag Spectrum, Gibbs Phenomenon and Hanning Smoothing  
The Lag Spectrum, Gibbs Phenomenon and Hanning Smoothing
        Continuum Observations in Line Mode  
Continuum Observations in Line Mode


    OBSERVATIONAL CONSIDERATIONS
OBSERVATIONAL CONSIDERATIONS
        Amplitude and Phase Calibration
Amplitude and Phase Calibration
        Bandpass Calibration
Bandpass Calibration
        Continuum Subtraction
Continuum Subtraction
        Interference
Interference
        Bandwidth and Time Smearing
Bandwidth and Time Smearing
        Determining the Observing Frequency
Determining the Observing Frequency
            Doppler tracking and dopset
Doppler tracking and dopset
                Velocity Definition
Velocity Definition
                Summing Velocities
Summing Velocities
                Velocity Rest Frame  
Velocity Rest Frame
            Running dopset
Running dopset
            Setting the LO Chain: loser  
Setting the LO Chain: loser
        System Temperature Corrections
System Temperature Corrections
            Changes In T$_{sys}$ With Elevation (T$_{spill}$)
Changes In T$_{sys}$ With Elevation (T$_{spill}$)
            Contributions to T$_{sys}$ From Strong Lines  
Contributions to T$_{sys}$ From Strong Lines


    REFERENCES
REFERENCES
    Bandwidth and Number of Channels
Bandwidth and Number of Channels
        Normal Mode
Normal Mode
        On-line Hanning Smoothing Option  
On-line Hanning Smoothing Option


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== VLA Spectral Line Observing ==  
= Introducing VLA Spectral Line Observing =


This guide is to help understand and set up spectral line observations at the VLA. The new, wide bandwidths of the VLA allow users to observe up to 8GHz of spectral bandwidth at a time. Apart from extreme continuum sensitivity, the wide bands of the VLA can be used to observe multiple spectral lines simultaneously. Furthermore, the WIDAR correlator is extremely flexible and acts, fundamentally, like up to 64 independent correlators with different bandwidths, channel numbers, polarization products, and observing frequencies. The final VLA can
The newly available wide bandwidths of the VLA allow users to observe up to 8GHz of bandwidth at a time. All observations with the upgraded VLA are inherently spectral observations, including those intended for continuum science.  The VLA's improved sensitivity and wide bandwidths greatly enhance the VLA's functionality for spectral line purposes, enabling simultaneous imaging of multiple spectral lines. The WIDAR correlator is extremely flexible and can act as up to 64 independent correlators with different bandwidths, channel numbers, polarization products, and observing frequencies. The final VLA will be able to


* deliver continuous spectral coverage up to a full width of 8GHz
* deliver continuous spectral coverage of up to 8GHz
* access 1GHz or 2GHz chunks in each receiver band (called basebands) and place correlator subbands on them; this enables observations of multiple spectral lines at once
* access 1GHz or 2GHz chunks in each receiver band (called basebands) and place multiple correlator subbands within them
* use up to 64 subbands at a time that are independently tunable, and can be configured in different spectral bandwidths, channel numbers, and number of polarization products
* place up to 64 independently tunable subbands within a baseband; these can be configured with different bandwidths, channel numbers, and polarization products
* derive the frequency from the velocity of a given spectral line (Doppler Setting)
* tune the baseband and subband frequencies according to the object's velocity with respect to the earth (Doppler Setting)  
* Dynamically schedule the observations to use the best weather conditions for high frequency, high scientific impact projects
* dynamically schedule observations to use the best weather conditions for high frequency, high scientific impact projects


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* Post-processing: The [http://casa.nrao.edu CASA] package is the main data reduction software for VLA and ALMA and contains cutting edge data reduction code with continuum and spectral line processing being the main focus. The software and reference guides can be obtained on the [http://casa.nrao.edu CASA homepage]. The [http://casaguides.nrao.edu CASAguides wiki] contains guides on VLA spectral line data reduction as well as some hints, tips and tricks on using CASA and the visualization tools that are designed to display spectral data cubes
* Post-processing: The [http://casa.nrao.edu CASA] package is the main data reduction software for VLA and ALMA and contains cutting edge data reduction code with continuum and spectral line processing being the main focus. The software and reference guides can be obtained on the [http://casa.nrao.edu CASA homepage]. The [http://casaguides.nrao.edu CASAguides wiki] contains guides on VLA spectral line data reduction as well as some hints, tips and tricks on using CASA and the visualization tools that are designed to display spectral data cubes
-->
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For the August 1 deadline, the following capabilities are offered (more options are available for "shared risk" observing, see the [https://science.nrao.edu/facilities/evla/docs/manuals/oss VLA Observational Status Summary]):
The following capabilities are offered for standard observing in the August 2012 proposal submission deadline.  Greater flexibility is available through "shared risk" observing, as discussed below and detailed in the [https://science.nrao.edu/facilities/evla/docs/manuals/oss VLA Observational Status Summary].


* Maximum data rates of 20MB/s
* Doppler setting
* 8-bit samplers providing
** two 1GHz basebands
** up to 16 independently tunable subbands per baseband
** independent number of polarization products in each subband
** independent subband bandwidths ranging from 31.25kHz to 128MHz
** independent number of channels in each subband; maximum number of channels distributed across all subbands and polarization products cannot exceed 16384 channels
* 3-bit samplers providing
** four 2GHz basebands with a total of 64 128MHz subbands. 2MHz resolution full polarization, 1MHz dual and 0.5MHz single polarization


* two polarization pairs 1GHz each (using the 8-bit samplers)
=  Line Frequencies and Velocity Conventions =
* up to 16 subband polarization pairs per baseband polarization pair
* independent tuning of all subbands
* independent subband bandwidth in the 31.5-128MHz range
* independent number of channels, up to 2048 channels for single polarization product, 1024 dual, 512 full polarization; maximum number of channels distributed across all subbands and polarization products cannot exceed 16384 channels
* Doppler setting
* The maximum data rate, however, cannot exceed 15MB/s
* for the wideband 3-bit samplers, 2x2GHz baseband polarization pairs can be set up with 2MHz resolution


== Line Rest Frequencies ==
== Line Rest Frequencies ==


There are a number of online tools available that help spectral line observers. The recommended line rest frequency catalog for VLA and ALMA is [http://splatalogue.net Splatalogue] which contains data from the [http://physics.nist.gov/cgi-bin/micro/table5/start.pl Lovas catalog], the [http://spec.jpl.nasa.gov/ JPL/NASA molecular database]], the [http://www.astro.uni-koeln.de/cdms/ Cologne Database for Molecular Spectroscopy], as well as radio recombination lines and data from other resources.
Spectral line calalogues available online are useful for selecting targeted line rest frequencies. The recommended catalog for VLA and ALMA observing is [http://splatalogue.net Splatalogue] which contains molecular line data from sources including the [http://physics.nist.gov/cgi-bin/micro/table5/start.pl Lovas catalog], the [http://spec.jpl.nasa.gov/ JPL/NASA molecular database]], the [http://www.astro.uni-koeln.de/cdms/ Cologne Database for Molecular Spectroscopy], as well as radio recombination lines.


== Observing Frequency and Velocity Definitions ==
== Observing Frequency and Velocity Definitions ==


The first step is to determine the observing frequency <math>\nu</math> of the spectral line. This is derived from the radial velocity <math>v</math> of the source and the rest frequency <math>\nu_0</math> of the spectral line.  
The frequency at which we must tune the correlator in order to observe a spectral line (<math> \nu </math>) is derived from the radial velocity of the source (v) and the rest frequency of the spectral line (<math> \nu_0 </math>). The ''relativistic velocity'', or true radial velocity, is related to the observed and rest frequencies by <math> v = \frac{\nu_0^{2} - \nu^{2}}{\nu_0^{2}+\nu^{2}}</math>. This equation is a bit cumbersome to use; in astronomy two different approximations are typically used:


A full relativistic calculation shows that the velocity <math>v</math> is determined via <math> v = \frac{\nu_0^{2} - \nu^{2}}{\nu_0^{2}+\nu^{2}}</math> and is called the ''relativistic velocity''. The equation is a bit cumbersome to use and in astronomy two different approximations are typically used instead:
* '''Optical Velocity''' <math> v^{optical} = \frac{\lambda-\lambda_0}{\lambda_0}\,\,c = cz </math> (<math>z</math> is the redshift of the source)


* '''Optical Velocity''' <math> v^{optical} = \frac{\lambda-\lambda_0}{\lambda_0}\,\,c = cz </math>    (<math>z</math> is the redshift of the source)
* '''Radio Velocity''' <math> v^{radio} = \frac{\nu_0-\nu}{\nu_0}\,\,c = \frac{\lambda-\lambda_0}{\lambda}\,\,c \neq v^{optical}</math>


* '''Radio Velocity''' <math> v^{radio} = \frac{\nu_0-\nu}{\nu_0}\,\,c = \frac{\lambda-\lambda_0}{\lambda}\,\,c \neq v^{optical}</math>  
The radio and optical velocities are not identical. Particularly,<math> v^{optical}</math> and <math> v^{radio}</math> diverge for large velocities. Optical velocities are predominantly used for extragalactic and radio velocities for Galactic targets.


At high redshifts, it is advisable to place the zero point of the velocity frame into the source via


The radio and optical velocities are not identical. At low velocities that difference is small but <math> v^{optical}</math> and  <math> v^{radio}</math> diverge more and more for large values. Traditionally, the optical velocities are predominantly used for extragalactic and the radio velocities for Galactic targets.
<math> \nu = \frac{\nu_0}{z+1} </math>,


At significant redshifts, it is advisable to place the zero point of the velocity frame into the source via
where the redshifted <math>\nu</math> can now be used as the input frequency for the observations. This method will appropriately apply the redshift correction to the channel and line widths and the resulting velocities are also intrinsic to the source.
 
<math> \nu = \frac{\nu_0}{z+1} </math>
 
where the redshifted <math>\nu</math> can now be used as the input rest frequency for the observations. The velocities that are derived based on such a redshifted rest frequency will be correctly scaled for the spread of the velocity scale that is caused by the redshift.


== Velocity Frames ==
== Velocity Frames ==


The earth rotates, revolves around the sun, rotates around the galaxy, moves within the Local Group, and shows motion against the cosmic microwave background. If one measures a velocity it is therefore necessary to correct for such motions and define the frame in which the velocities are measured.  
The earth rotates, revolves around the sun, rotates around the galaxy, moves within the Local Group, and shows motion against the cosmic microwave background. Any source velocity must therefore always be specified relative to a reference frame.  


Various velocity rest frames are used in the literature. The following table lists their name, the motion that is corrected for, and the maximum amplitude of the velocity correction. Each rest frame correction is incremental to the preceding row:


There are various rest frames used in the literature. The following table lists their name, the motion that is corrected for, and the maximum amplitude of the velocity correction. Each rest frame correction is incremental to the preceding row:


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The velocity frame should be chosen based on the science. For most observations, however, one of the the following three reference frames is commonly used:
The velocity frame should be chosen based on the science. For most observations, however, one of the following three reference frames is commonly used:
 
<br>
* '''Topocentric''', the natural velocity frame of the observatory (defining the sky frequency of the observations). Visibilities in a measurement set are typically stored in this velocity frame.


* '''Local Standard of Rest''': the native output of images in CASA. Note that there are two varieties of LSR: the kinematic LSR ('''LSRK''') and the dynamic (LSRD) definitions for the kinematic and dynamic centers, respectively. In almost all cases LSRK is being used and the less precise LSR naming is usually used synonymously with more modern LSRK definition.  
* '''Topocentric''' is the velocity frame of the observatory (defining the sky frequency of the observations). Visibilities in a measurement set are typically stored in this velocity frame.


* '''Barycentric''': a common frame that has virtually replaced the older heliocentric standard. Given the small difference between barycentric and heliocentric, they were frequently used interchangeably.  
* '''Local Standard of Rest''' is the native output of images in CASA. Note that there are two varieties of LSR: the kinematic LSR ('''LSRK''') and the dynamic (LSRD) definitions for the kinematic and dynamic centers, respectively. In almost all cases LSRK is being used and the less precise name 'LSR' is usually used synonymously with more modern LSRK definition.


* '''Barycentric''' is a commonly used frame that has virtually replaced the older heliocentric standard. Given the small difference between the barycentric and heliocentric frames, they were frequently used interchangeably.


A full list of CASA supported reference frames is provided in the [[http://casa.nrao.edu/docs/userman/UserMan.html CASA reference Manual and Cookbook]] and also on the [[http://casaguides.nrao.edu/index.php?title=Velocity_Reference_Frames casaguides.nrao.edu webpage]]
<br>
A full list of CASA supported reference frames is provided in the [http://casa.nrao.edu/docs/userman/UserMan.html CASA reference Manual and Cookbook] and also on the [http://casaguides.nrao.edu/index.php?title=Velocity_Reference_Frames casaguides.nrao.edu webpage]


== Doppler Correction ==
== Doppler Correction ==


A telescope frequently operates at a fixed sky frequency (Topocentric velocity frame). Any spectral line will thus shift during any observation campaign. Within a day, the rotation of the earth dominates and the line may shift up to <math>\pm</math>0.5km/s, depending on the position on the sky (see above). Observing campaigns that span over a year, may see spectral lines to shift in frequency by up to <math>\pm</math>30km/s due to the earth's motion around the sun.  
A telescope naturally operates at a fixed sky frequency (Topocentric velocity frame) which can be adjusted to account for the motion of the earth. A spectral line's observed frequency will shift during any observing campaign. Within a day, the rotation of the earth dominates and the line may shift up to <math>\pm</math>0.5km/s, depending on the position of the source on the sky (see above). Observing campaigns that span a year may have spectral lines that shift by up to <math>\pm</math>30km/s due to the earth's motion around the sun.


<i>Side note: As a rule of thumb, 1 MHz in frequency corresponds roughly to <math>x</math> km/s for the line at a wavelength of <math>x</math> in mm. E.g., at a wavelength of 7mm, 1MHz corresponds to about 7km/s in velocity, at 21cm 1MHz corresponds roughly to 210km/s. </i>
<i>Note: As a rule of thumb, 1 MHz in frequency corresponds roughly to <math>x</math> km/s for the line at a wavelength of <math>x</math> in mm. E.g., at a wavelength of 7mm, 1MHz corresponds to about 7km/s in velocity, at 21cm 1MHz corresponds roughly to 210km/s. </i>


Using this rule of thumb, a line may shift by up to <math>\pm</math>5MHz in Q-band and by up to <math>\pm</math>0.15MHz in L-band over the course of a year. This needs to be taken into account when setting up the observations. There are different ways to do this:  
Using this rule of thumb, a line may shift by up to <math>\pm</math>5MHz in Q-band and by up to <math>\pm</math>0.15MHz in L-band over the course of a year. This needs to be taken into account when setting up the observations. This issue can be handled in different ways:


* use the same sky frequency for all observations. The shift of the line (maximum of <math>\pm</math>30km/s) is accommodated by a wide bandwidth that covers the line and its width at any time of the observation campaign. The data is later regridded in CASA to a common LSRK or BARY velocity frame. The sky frequency of an observation can be computed with the [http://www.vla.nrao.edu/astro/guides/dopset/ Dopset tool] for a given time. One may find the LST dates for an observation on the [http://www.vla.nrao.edu/cgi-bin/schedules.cgi VLA Schedule Page].
* use the same sky frequency for all observations, accommodating the line shift (maximum of <math>\pm</math>30km/s) by using a wide enough bandwidth to cover the line at any time in the observing campaign. The data is later regridded in CASA to a common LSRK or BARY velocity frame. The sky frequency of an observation can be computed with the [http://www.vla.nrao.edu/astro/guides/dopset/ Dopset tool] for a given time. One may find the LST dates for an observation on the [http://www.vla.nrao.edu/cgi-bin/schedules.cgi VLA Schedule Page].


* calculate a fixed sky frequency at the beginning of an observation, then keep it fixed for the duration of the scheduling block. This is called '''Doppler Setting''' and offered by OPT (currently only in OSRO mode). The line shift is then minimized to the rotation of the earth (maximum amplitude <math>\pm</math>0.5km/s). This small shift is corrected by post processing.  
* calculate the sky frequency at the beginning of an observing block and keep this fixed for the duration of the scheduling block. This is called '''Doppler Setting''' and offered by OPT '''for each baseband''' (currently only in OSRO mode). The line shift is then reduced to the rotation of the earth (maximum amplitude <math>\pm</math>0.5km/s). This small shift is corrected in data processing.


* change the sky frequency continuously to keep the line at the same position in the band. This method is called '''Doppler tracking''' and was standard for the pre-upgrade VLA. <i>The new VLA does NOT support Doppler tracking.</i> The WIDAR correlator offers enough bandwidth and spectral channels to cover any line shift and post-processing regridding needs. In addition, a non-variable sky frequency may also deliver a more robust calibration and overall system stability.  
* change the sky frequency continuously to keep the line at the same position in the band. This method is called '''Doppler tracking''' and was standard for the pre-upgrade VLA. <i>The new VLA does NOT support Doppler tracking.</i> The WIDAR correlator offers enough bandwidth and spectral channels to cover any line shift and post-processing regridding needs. In addition, a non-variable sky frequency may also deliver a more robust calibration and overall system stability.


The post-processing regridding of the line in CASA can be either done directly during imaging in the task <tt>clean</tt>, or alternatively with the task <tt>cvel</tt>. The regridding works well when the spectral features are sampled with at least 4 channels.
The regridding of the spectrum can be completed during data processing in CASA, either directly during imaging in the task <tt>clean</tt>, or alternatively with the task <tt>cvel</tt>. The regridding works well when the spectral features are sampled with at least 4 channels.


== Gibbs Phenomenon and Hanning smoothing ==
= The WIDAR Correlator =


For very sharp spectral or lag features, a Fourier transform can prominently display a sinc function, a channel by channel fluctuation of the amplitude. If this is apparent in the data, smoothing adjacent channels will reduce or even eliminate the effect. The smoothing kernel to be used is a Hanning smoothing function which sports a triangular kernel with the central channel being weighed by 0.5 and the two adjacent channels by 0.25. After Hanning smoothing, however, the channels are not independent anymore and one can eliminate every other channel without losing signal to noise.


In the VLA days, the correlator design (with a truncated lag spectrum) could show the Gibbs phenomenon relatively prominently and Hanning smoothing was frequently applied online during the observations to accommodate for the effect and to save disk space. The Gibbs phenomenon is much less common for the VLA due to a better correlator design of WIDAR. Only very strong maser or RFI sources may exhibit the typical "ringing" feature of the Gibbs phenomenon. In addition, data can be stored rather cheaply so there is no need for data size reduction via online Hanning smoothing anymore. Consequently, the VLA does not support online Hanning smoothing. If Hanning smoothing is required, it has to be performed in post-processing, e.g. with the CASA task [http://casa.nrao.edu/docs/TaskRef/hanningsmooth-task.html <i>hanningsmooth</i>].
== Basebands ==


== Sensitivity/Exposure Time Calculation ==
Let's start with the basics: A signal from the telescope enters WIDAR, is split into its left and right hand circular polarizations, and passes through analog filters that define the <b>basebands</b>.  Basebands set the spectral range that can be accessed by subbands, and they come in baseband pairs to cover L and R polarizations. Each baseband pair can be set to one baseband sky frequency. Basebands are the most fundamental spectral ranges delivered from the samplers and digitizers to the correlator. With 8-bit sampling, the samplers deliver two independently tuneable baseband pairs (dubbed A0/C0 and B0/D0) with 1 GHz bandwidth each. 3-bit sampling provides four baseband pairs (A1/C1, A2/C2, B1/D1, B2/D2), each of them 2GHz wide.
[[File:WIDARcorrelatorbands.png|300px|thumb|right|Baseband with WIDAR subbands]]


[[File:ExposureCalculator.png|300px|thumb|right|The VLA Exposure Calculator]]
=== Baseband Tuning Restrictions ===


The sensitivity of spectral line data is best calculated with the [https://science.nrao.edu/facilities/evla/calibration-and-tools/exposure VLA Exposure Calculator]. This JAVA tool (see screenshot) allows one to enter sensitivity limits and provides the required time on source given a frequency, weather, weighting scheme, polarization products, and bandwidth of the observations. The '''Bandwidth''' should correspond to the frequency resolution that is required to perform the science. This may or may not be the width of individual spectral channels. Overheads need to be added according to our [https://science.nrao.edu/facilities/evla/proposing/frequently-asked-questions VLA frequently asked questions webpage]. We  refer to the [http://evlaguides.nrao.edu/index.php?title=Category:LowFrequency low frequency guide] and [http://evlaguides.nrao.edu/index.php?title=Category:HighFrequency high frequency guide] for further advice on how to set up the observations, depending on the receiver band to be used.
The following restrictions apply to baseband tuning:


== The WIDAR Correlator ==
* With 8-bit sampling in Ka band, only one baseband can be below 32GHz and that must be B0/D0
* 3-bit samplers can only be used in C-band or above, where the instantaneous frequency width of the receiver is larger than 2GHz.
* The 3-bit A1C1 baseband frequency can have a separation of max. 4GHz from the A2D2 baseband. B1D1 and B2D2 have the same restriction of 4GHz maximum separation


Together with the wide, instantaneous receiver bandwidths, the WIDAR correlator at the VLA is very flexible and provides a number of setup options that are relevant for spectral line observing.
== Fixed 128MHz Subbands and 128MHz "Suckouts" ==


=== Basebands ===
After filtering through the basebands, the signal enters the correlator and is split into fixed, 128MHz wide subbands. They are placed adjacently to cover the full width of the basebands.  Narrow subbands can be arranged within these fixed 128MHz subbands. Because each fixed 128MHz subband has a filter shape with soft corners, the sensitivity of the VLA drops to about half its maximum value between any two fixed 128MHz subbands. These frequency ranges are called "128 MHz Suckouts". There are two primary options for dealing with the suckouts:


Let's start with the basics: A signal from the telescope enters the WIDAR and along that path, it is passing analog filters that define the <b>basebands</b>. The basebands are actually baseband pairs to cover the L and R polarizations. They are the most fundamental spectral ranges delivered from the samplers and digitizers to the correlator. In the 8-bit mode, the samplers deliver two independably tuneable basebands (dubbed AC0 and BD0) with 1 GHz bandwidth each. Using the 3-bit samplers, there are 4 baseband pairs (A1C1, A2C2, B1D1, B2D2), each of them 2GHz wide. Each baseband pair can be set to a baseband sky frequency. Alternatively, the sky frequency can be calculated using the Doppler Setting tool in OPT.  
1. Try to set the baseband frequency such that targeted lines do not fall in the suckouts. We offer the [https://e2e.nrao.edu/tune.shtml spectral line setup tool "TUNE"] that can be used to maximize the frequency separation between a number of spectral lines and the suckouts.  


When the signal enters WIDAR, the basebands pass through a number of digital filters, each 128MHz wide, the fixed 128MHz subbands.


[[File:WIDARcorrelatorbands.png|300px|thumb|right|Baseband with WIDAR subbands]]
2. If it is not possible to obtain coverage of all of your lines using the above method, observe with two basebands shifted by 10-64MHz apart. This will ensure that at one baseband covers the suckouts of the other baseband with full sensitivity. An example is given in the figures to the right.  
 
=== Fixed 128MHz Subbands and 128MHz "Suckouts" ===
 
After filtering through the basebands, the signal enters the correlator and is split into fixed, 128MHz wide subbands. They are placed adjacently to cover the full width of the basebands. As each fixed 128MHz subband has some filter shape with soft corners, the sensitivity of the VLA drops to about half its value between any two fixed 128MHz subbands. These frequency ranges are called "128 MHz Suckouts". There are a few options to account and interpolate over the suckouts:
 
* Easiest method is to avoid them in your spectral setup. Try to set the baseband frequency in a way such that any interesting lines do not fall in the suckouts. We offer a [https://webtest.aoc.nrao.edu/cgi-internal/nroy/work.py spectral line setup tool] that can be used to maximize the frequency separation between a number of spectral lines and the suckouts. 
 
 
* Observe with two basebands shifted by 10-64MHz apart. This will ensure that at one of the baseband covers the suckouts of the other baseband with full sensitivity. An example is given in the figures to the right. To obtain more spectral channels, one can consider to using single polarizations on the basebands.  




Line 182: Line 179:
rms noise in a blank field as a function of frequency for two basebands consisting of 8 contiguous sub-bands, where the basebands are separated by one-half of the subband width. Wherever signal in one baseband is compromised by edge effects, data from the other subband are substituted.]]
rms noise in a blank field as a function of frequency for two basebands consisting of 8 contiguous sub-bands, where the basebands are separated by one-half of the subband width. Wherever signal in one baseband is compromised by edge effects, data from the other subband are substituted.]]


=== Narrow Subbands ===  
== Correlator Resources and Subband Placement ==


For every baseband, there can be a maximum of 64 subbands (for the August 2012 deadline, we offer up to 16 subbands per baseband for normal observations). Fundamentally, each narrow subband can be between 128 MHz and 31.25 kHz wide and contains 64 channels when all four RR, LL, RL, LR polarization products are required (full polarization), 128 channel in dual polariation mode (RR & LL), and 256 channels for single, RR or LL polarization products. The two tables below display the options and the corresponding velocity widths and channelizations:
Correlator baselineboards (BlBs, also named "BL.BPS" for "baselineboard pairs" in the OPT) are independent hardware units that are allocated to narrow subbands.  <b>WIDAR has 64 baselineboard pairs.</b> As a result, WIDAR supports a maximum of 64 subband pairs (again, pairs to cover the two polarizations), but the number of subbands depends on the subband setups as described below. For the same number of channels, single (RR or LL), dual (RR & LL), and full (RR, LL, RL, and LR) polarization products require 1, 2, and 4 times the correlator baselineboards respectively. Similarly, doubling the number of channels in a subband doubles the number of correlator resources used. 


* Full polarization
=== Narrow Subbands with the 8-bit sampler ===
Narrow subbands define the frequency ranges in which the spectrum is measured.  Narrow subbands with bandwidths between 31.25kHz and 128MHz can be arranged to obtain desired frequency coverage and spectral resolution within the baseband.  Each narrow subband needs to be entirely within a fixed 128MHz subband, as <b> narrow subbands cannot cross a 128MHz suckout frequency.</b>  The baseband center frequency can be shifted and subband bandwidths and frequencies must be arranged to avoid the suckouts. This implies that the 128MHz fixed subbands cannot be moved as they would fall on a suckout at any frequency offset from a 128MHz "raster" within the baseband. All subbands less than 128MHz in width, however, and can be independently tuned as long as they do not cross a suckout. Furthermore, <i>all subbands can be set up with different bandwidths, frequency resolutions, channel numbers, and polarization products.</i>
 
=== Standard Subbands ===
 
Standard subbands allocate a single baselineboard pair to a each subband. Standard subbands contain 64 channels when full polarization (RR,LL, RL & LR) products are required, 128 channels in dual polarization mode (RR & LL), and 256 channels for single polarization (RR or LL).  Options for full and dual polarization subbands, with frequency and velocity resolutions, are shown in the following tables.  For the August 2012 deadline, we offer up to 16 subbands per baseband for normal observations, and up to 64 for shared risk observations.
 
* Full polarization


<table class="grid listing" border="1">
<table class="grid listing" border="1">
<tr valign="top">
<tr valign="top">
<th>Sub-band BW (MHz) </th><th>Number of channels/poln product </th><th>Channel width (kHz) </th><th>Channel width (km/s at 1 GHz) </th><th>Total velocity coverage per sub-band                       (km/s at 1 GHz) </th>
<th>Sub-band BW (MHz) </th><th>Number of channels/poln product </th><th>Channel width (kHz) </th><th>Channel width (km/s at 1 GHz) </th><th>Total velocity coverage per sub-band (km/s at 1 GHz) </th>
</tr>
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<table class="grid listing" border="1">
<table class="grid listing" border="1">
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<th>Sub-band BW (MHz) </th><th>Number of channels/poln product </th><th>Channel width (kHz) </th><th>Channel width (km/s at 1 GHz) </th><th>Total velocity coverage (km/s at 1                       GHz) </th>
<th>Sub-band BW (MHz) </th><th>Number of channels/poln product </th><th>Channel width (kHz) </th><th>Channel width (km/s at 1 GHz) </th><th>Total velocity coverage (km/s at 1 GHz) </th>
</tr>
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</table>
</table>


=== Baselineboard Stacking ===


In order to obtain a larger number of channels per subband, a method called "baselineboard stacking" can be used.  Baselineboard stacking allows a larger number of baseline boards to be allocated to a single subband, increasing the number of channels within the subband.  To double the number of channels in a subband, simply double the number of baselineboards allocated to the subband.  Thus each additional BlB for a subband adds another 64 channels in full and 128 channels in dual, and 256 channels in single polarization modes.


As an example, the full 2GHz bandwidth of the 8-bit samplers can be covered by 16 128MHz standard narrow subbands, each with 128 channels in dual polarization, as shown in the table above. The 16 subbands, however, only require 16 BlBs and another 48 are available for baselineboard stacking. One can thus use 4 BlBs for each of the subbands, quadrupling the number of channels from 128 to 512, which reduces the channel widths from 1MHz to 0.25 MHz over the full 2 GHz frequency range. This method works for any subband bandwidth. 


Baselineboard stacking can be very useful for spectral line work, as it allows for wider bandwidths for each subband while maintaining frequency resolution. With baselineboard stacking, you can use fewer subbands to cover a set frequency range, thereby minimizing the number of filter edges and resulting sensitivity dropoff.  Baselineboard stacking is offered for the August 2012 deadline. For shared risk observations, however, we recommend that observers do not use all 64 baselineboards to allow for observing in the event that one or two baselineboards are not working on any given day.


<!-- JUERGEN - might be helpful to have a table that shows the # of channels in a single, dual, and full polarization subband and the # of baselineboard pairs allocated -->


==== WIDAR Tuning Restrictions ====


The baseband (pairs) cannot be entirely independently tuned. The following restrictions apply:
=== Recirculation ===


* 3-bit samplers can only be used in C-band or above, where the instantaneous frequency width of the receiver is larger than 2GHz.
"Recirculation" is a second method for obtaining more spectral channels in a given subband. Recirculation uses the fact that the correlator has more computing capability when the data is averaged in time. The standard correlator dump time is 1s. If this is doubled to 2s, WIDAR can produce twice as many channels as listed in the tables above. 4s would allow 4 times the number of channels. Recirculation is only available for shared risk observations for the August 2012 proposal deadline.
* In Ka band, 3-bit samplers, only one baseband can be below 32GHz and that must be BD
* The 8-bit A1C1 baseband frequency can have a separation of max. 4GHz from the A2D2 baseband. B1D1 and B2D2 have the same restriction of 4GHz maximum separation


For the narrow subbands:
== Narrow Subbands with the 3-bit sampler ==


* Narrow subbands can not go across a 128MHz suckout frequency. Any narrow subband needs to be entirely within a 128MHx fixed subband.
As the 3-bit samplers are still under commissioning, the August 2012 proposal deadline offers only a single observing setup with 3-bit sampling.  This setup includes full coverage over all four baseband pairs (using 64 128MHz subband) 2MHz resolution for full polarization, 1MHz dual, and 0.5MHz for single polarization.


'''(??? Michael to comment - what other restrictions are there ???)'''
== Data Rate Limits ==


=== Correlator Baselineboards ===
Baselineboard stacking, recirculation, and time resolution can add up to an extremely high data rate in the correlator. The VLA currently supports data rates of up to 20MB/s for regular and more for shared risk observing, see the [https://science.nrao.edu/facilities/evla/docs/manuals/oss Observational Status Summary for details]. The OPT instrument configuration calculates data rates based on the spectral line setup and the data rate maxima should not be exceeded for any observational setup.


Correlator baselineboards (BlBs) are independent units that can be used for separate subbands. WIDAR has 64 BlB pairs (for the polarizations) and thus supports a maximum of 64 subbands.
= Tips for Planning, Setup, and Processing of VLA Spectral Line Observations =


=== Baselineboard Stacking ===
'''Reminder:''' The following capabilities are offered for regular observing for the August 1 2012 proposal deadline.  Consult the [https://science.nrao.edu/facilities/evla/docs/manuals/oss VLA Observational Status Summary] for additional options available through shared risk observing:


If not all of the 64 subbands are used, the remaining BlBs can be used to obtain more channels per subband. This method is called "baselineboard stacking" and each additional BlB for a subband adds another 64 channels in full and 128 channels in dual polarization modes. In OPT's resource tool (RCT), this can be set via the BlB.BPS dropdown menu that is available for each subband. Baselineboard stacking is extremely useful as it allows to use wider bandwidths in each subband but yet maintain a high number of channels. This minimizes the number of filter edges that are unavoidable for each subband. Alternatively, for a given subband bandwidth, BlB stacking provides a higher spectral resolution. E.g. the full 2GHz bandwidth of the 8bit samplers can be covered by 16 128MHz subbands, each with 128 channels dual polarization as of the table above. The 16 subbands, however, only require 16 BlBs and another 48 are available for baselineboard stacking. One can thus use 4 BlBs for each of the subbands, quadrupling the number of channels from 128 to 512, which reducies the channel widths from 1MHz to 0.25 MHz over the full 2 GHz frequency range. This method works for any subband bandwidth.


=== Recirculation ===
* Maximum data rates of 20MB/s
* Doppler setting
* 8-bit samplers providing
** two 1GHz basebands
** up to 16 independently tunable subbands per baseband
** independent number of polarization products in each subband
** independent subband bandwidths ranging from 31.25kHz to 128MHz
** independent number of channels in each subband; maximum number of channels distributed across all subbands and polarization products cannot exceed 16384 channels
* 3-bit samplers providing
** four 2GHz basebands with a total of 64 128MHz subbands. 2MHz resolution full polarization, 1MHz dual and 0.5MHz single polarization


Another way to obtain more spectral channels for a given subband is called "recirculation". Recirculation uses the fact that the correlator has more computing capability when the data is averaged in time. The basic correlator dump time is 1s. If this is doubled to 2s, WIDAR can produce twice as many channels as listed in the tables above. 4s would allow 4 times the number of channels. This setup, however, is only available in for shared risk observations.


=== Data Rate Limits ===
== Considerations for Planning Subband Bandwidths and Resolution ==


Baselineboard stacking, recirculation, and time resolution, however, can add up to an extremely high data rate in the correlator. The VLA currently supports data rates of up to 15MB/s for regular and higher data rates for shared risk observing. The OPT instrument configuration calculates data rates based on the spectral line setup and the limit of 65 MB/s should not be exceeded for any observational setup.
=== Bandwidths required for UV continuum subtraction ===


== Planning and Setup of VLA Spectral Line Observations==
When determining the bandwidths needed in your subbands, it is important to observe enough line-free channels on each side of the spectral line to allow for good continuum subtraction. It is possible to interpolate the continuum levels from one subband to another, but it is usually a better solution to derive the continuum level in each subband separately. The number of line free channels ideally equals or exceeds the number of channels that cover the line. When the line free channels are distributed equally on both sides of the spectral line, a low order polynomial (polynomials of order 1 appear to be good models for most cases) usually provides a good fit. Whenever higher order polynomials are needed, e.g. for a continuum source with a significant spectral curvature over the subband(s), the number of line-free channels should be increased.


=== Channel Widths for High Dynamic Range Imaging: Dealing with the Gibbs Phenomenon ===


For the August 1 deadline, the following capabilities are offered (more options are available for "shared risk" observing, see the [https://science.nrao.edu/facilities/evla/docs/manuals/oss VLA Observational Status Summary]):
For very sharp spectral or lag features, the spectrum can prominently display a ringing effect known as the Gibbs phenomenon, a sinc function that zig-zags on alternating channels. If this is apparent in the data, smoothing adjacent channels will reduce or even eliminate the effect. Hanning smoothing is the most effective method, which uses a triangular smoothing kernel with the central channel weighed by 0.5 and the two adjacent channels by 0.25. After Hanning smoothing, however, the channels are not independent anymore and one can eliminate every other channel without losing signal to noise.


* up to 16 subband pairs per baseband pair
In the pre-upgrade VLA days, the correlator design had a realtively short, truncated lag spectrum, which could result in prominent Gibbs ringing. To avoid this effect, Hanning smoothing was frequently applied online during the observations. With the new WIDAR capabilities of the upgraded VLA, however, ringing is very rare and only observed for extremely strong maser or RFI sources. Consequently, the VLA does not support online Hanning smoothing anymore; if required, Hanning smoothing can be applied during post-processing (e.g. with the CASA task [http://casa.nrao.edu/docs/TaskRef/hanningsmooth-task.html <i>hanningsmooth</i>].). The Gibbs effect can also be reduced by using higher spectral resolution that covers the spectral feature with several channels. In that case, the ringing effects from the individual channels beat against each other, effectively reducing the zig-zag pattern that appears on alternating, neighboring channels when the peak is within a single spectral channel.
* independent tuning of all subbands
* independent subband bandwidth in the 31.5-128MHz range
* independent number of channels, up to 2048 channels for single polarization product, 1024 dual, 512 full polarization; maximum number of channels distributed across all subbands an polarization products cannot exceed 16384 channels
* Doppler setting
* The maximum data rate, however, cannot exceed 15MB/s


== Sensitivity/Exposure Time Calculation ==


[[File:ExposureCalculator.png|300px|thumb|right|The VLA Exposure Calculator]]


=== The Proposal Submission Tool (PST) ===
After planning your general setup and sensitivity needs, the required on-source observing time is best calculated with the [https://science.nrao.edu/facilities/evla/calibration-and-tools/exposure VLA Exposure Calculator]. This JAVA tool (see screenshot) allows one to input the required rms noise and bandwidth limits and outputs the required time on source given a frequency, weather, weighting scheme, and number of polarization products. The input '''Bandwidth''' should correspond to the frequency resolution that is required to perform the science. This may or may not be the width of individual spectral channels. Overheads need to be added according to our [https://science.nrao.edu/facilities/evla/proposing/frequently-asked-questions VLA frequently asked questions webpage]. We refer to the [http://evlaguides.nrao.edu/index.php?title=Category:LowFrequency low frequency guide] and [http://evlaguides.nrao.edu/index.php?title=Category:HighFrequency high frequency guide] for further advice on how to set up the observations, depending on the receiver band to be used.
 
To observe all the restrictions listed above for the August 2012 deadline, the Proposal Submission Tool (PST) includes a spectral setup tool. The PST is accessible via [http://my.nrao.edu my.nrao.edu]. You may need to register at if you do not yet have an account.


[[File:PST-wide.png|200px|thumb|right|Snapshot of the 3-bit sampler, wideband mode PST setup tool]]
== The Proposal Submission Tool (PST) == 


The Proposal Submission Tool (PST, accessible at [http://my.nrao.edu my.nrao.edu] ) is used to submit proposals, including the scientific justification, abstract, and authors, as well as planned target sources, observing session lengths, and correlator setups.  In order to ensure that planned correlator setups comply with the capabilities offered for the August 2012 deadline, the Proposal Submission Tool (PST) includes a spectral setup tool.


Wide-band mode (3-bit sampler, up to 8GHz bandwidth) can be used for spectral line observing as well. The channel width is fixed to a resolution of 2MHz and no narrow subbands can be chosen.
<!-- link to Michaels docs-->


[[File:PST-narrow.png|200px|thumb|right|Snapshot of the 8-bit sampler, 2x1GHz PST setup tool]]
[[File:PST-narrow.png|200px|thumb|right|Snapshot of the 8-bit sampler, 2x1GHz PST setup tool]]


The 8-bit sampler data can be setup with the flexibility mentioned above. The figure shows an example where 9 subbands were chosen in Ka band, four in the first and 5 in the second baseband. The setup features different subband bandwidths, polarization products and baselineboard stacking (up to 16 for a couple of subbands). A total of 49 baseline boards are used for this configuration. A full description of the tool '''is provided here --- ??? link to Michael's manual'''.
In the example to the right, 9 subbands were chosen in Ka band with 8-bit sampling.  Four of the subbands are in the first and five in the second baseband. The setup features different subband bandwidths, polarization products and uses baselineboard stacking (up to 16 baseline board pairs per subband pair are used for a couple of subbands). A total of 49 baseline boards are used for this configuration.  


=== Setting up a Spectral Observation using the Observation Preparation Tool (OPT) ===


The Observation Preparation Tool (OPT) is the web-based interface to create scheduling blocks (SBs) for the VLA. An SB is a block of time during which an observation is executed. A full project may constitute of several SBs. To access the OPT, go to [http://my.nrao.edu my.nrao.edu] and click on the ''Obs Prep'' tab, followed by [https://e2e.nrao.edu/opt/ ''Login to the Observation Preparation Tool''].  Please also read the [https://science.nrao.edu/facilities/evla/observing/opt OPT webpage] that contains the latest information on the OPT as well as a comprehensive OPT manual.
[[File:PST-wide.png|200px|thumb|right|Snapshot of the 3-bit sampler, wideband mode PST setup tool]]


Wide-band mode (3-bit sampler, up to 8GHz bandwidth) can be used for spectral line observing as well. The channel width is fixed to a resolution of 2MHz for full, 1MHz for dual and 0.5MHz for single polarization products. No narrow subbands can be chosen.


==== Frequency Setup ====
== Setting up a Spectral Observation using the Observation Preparation Tool (OPT) ==


'''For the data taken early 2013 (August 2012 deadline), we will provide a specific new OPT Instrument Configuration layout for the regular and shared risk observing modes. The description below is for the current OSRO and RSRO modes.'''
The Observation Preparation Tool (OPT) is the web-based interface to create scheduling blocks (SBs) for time awarded on the VLA. An SB is the observing program used for a single observing run.  This consists of at least a few startup scans, a pointing reference, a bandpass calibration, a flux calibration, gain calibration and target observations.  In the OPT, you specify your sources, scan lengths and order, and correlator setups.  A full project may consist of several SBs.  To access the OPT, go to [http://my.nrao.edu my.nrao.edu] and click on the ''Obs Prep'' tab, followed by [https://e2e.nrao.edu/opt/ ''Login to the Observation Preparation Tool'']. Instructions for using the OPT and for selecting appropriate calibrators are provided in the [http://evlaguides.nrao.edu/index.php?title=Category:OPT-QuickStart OPT QuickStart Guide], and a comprehensive user's manual and up to date information on the OPT are available at [https://science.nrao.edu/facilities/evla/observing/opt OPT webpage]. As such, this guide provides only brief notes on the bandpass and gain calibrators and then focuses on the task of setting up correlator resources.


One of the most important parts of the setup is to chose the appropriate frequency setup for WIDAR. In the OPT, click on the ''Instrument Configuration'' tab. The most advanced setting is currently the RSRO setting (''File -> Create New -> RSRO Configuration''). This opens a page for the frequency setup as shown in the figures. Note that you may only hve access to the OSRO configuration utility, depending on your history of VLA observations. The OSRO setup is a more restricted version of the RSRO setup. 
=== Bandpass Setup ===


* Enter a name and select the receiver in the top panel. This will adjust the available frequency range described below the drop down menu. The 1dB and 3dB ranges describe the roll-off behavior of the receiver sensitivity at the receiver band edges. If the frequency to be observed is close to the edge frequency of a receiver, one may check if the next higher or lower frequency receiver is more suitable.
All observations with the VLA - even those with the goal of observing continuum - require bandpass calibration. When scheduling the bandpass calibration scans within an SB, the observer should be careful to minimize the number of shadowed antennas, as an antenna without a bandpass determined for it will essentially be flagged in the data for the rest of the observation. A bandpass calibrator should be bright enough, or observed long enough, so that the bandpass calibration does not significantly contribute to the noise in the image. This implies that, for a bandpass calibrator with flux density S<sub>cal</sub> observed for a time t<sub>cal</sub> and a science target with flux density S<sub>obj</sub> observed for a time t<sub>obj</sub>, <math> S_{cal} \sqrt{t_{cal}}</math> should be greater than <math>S_{obj} \sqrt{t_{obj}}</math>. How many times greater will be determined by one's science goals and the practicalities of the observations, but <math> S_{cal} \sqrt{t_{cal}}</math> should be greater by at least a factor of two. For extremely narrow channels or very weak bandpass calibrators, those typical flux requirements can lead to extremely large integration times. As an alternative one may then chose to reduce the integration time and interpolate in frequency, or to fit a polynomial across all channels in post-processing (''bandtype=BPOLY'' in CASA's [http://casa.nrao.edu/docs/TaskRef/bandpass-task.html bandpass task].


* ''Baseband Tuning'': Select the position of the basebands. For the ''8bit samplers'', the central frequencies for 2 basebands are to be provided, each with a width of 1GHz. The center frequencies will go into the A0C0 box for frequency 1 and into the B0D0 box for frequency 2. For ''3bit samplers'', 4 basebands are available, each 2 GHz wide. Here, the center frequencies need to placed into the A1C1, A2C2, B1D1, and B2D2 boxes. A1C1 and A2C2 cannot be more than 4GHz apart, and the same restrictions apply to B1D1 and B2D2. Note that additional frequency restrictions may apply and the OPT/ICT will issue a clear warning or error message for those cases. The graphical panel above the input boxes shows the position of the two or four basebands. They are displayed as pairs to accommodate the R and L polarization inputs that may later be converted into single, dual, or full polarization products.
The bandpass calibrator should be a point source or have a well-known model. At low frequencies, the absolute flux density calibrators (3C48, 3C147, or 3C286) are quite strong and can often double as bandpass calibrators. At high frequencies (Ku, K, Ka, Q), however, these sources have only moderate flux densities of ~0.5-3 Jy, translating into a potentially noisy bandpass solution. A different, stronger bandpass calibrator should then be observed. <!-- JUERGEN - can you suggest any --> Naturally, all of the above depends on the channel widths and for wide channels the standard flux calibrators may be sufficient even at higher frequencies. In turn, extremely narrow channels may require stronger bandpass calibrators at the low frequency end. Additionally, We have shown that one can transfer the bandpass from a wide subband onto a narrow subband if the wide bandpass frequency range covers that narrow one. This may be good to a level of a few per cent, but we advise to use that option only when absolutely necessary.


* ''Integration Time:'' this defines how data is dumped from the correlator backend into data files. A larger integration time will reduce the data volume. In addition, larger values can be chosen to take advantage of recirculation (only shared risk). On the other hand, time smearing effects, RFI excision, or time resolution may demand smaller integration times. It is important though, to not exceed the maximum data rate of 75 MB/s (15MB/s for regular observing) and the integration time parameter is a good way to stay below this threshold for observations that demand large number of spectral channels.
The stability of bandpasses as a function of time is of concern for high-dynamic-range spectral work. We have found that most antennas show bandpasses that are stable to a few (~2-4) parts in a thousand over a period of several (~4-8) hours. This should be sufficient for most scientific goals but the bandpasses can be observed several times during an observation for extreme calibration accuracy requirements.


* The total data rates are displayed in the ''Configuration Summary''. The same panel also shows the number of baseline boards that are used in the setup. Remember that a maximum of 64 baseline boards are available. During commissioning, it is advisable to not use all baseline boards, as some of them  may undergo maintenance. Also make sure to stay within the data rate restriction limits.
A complication can occur when the frequency range of the bandpass is contaminated by other spectral features such as RFI lines or Galactic HI in absorption or emission. There are two basic options to accommodate that situation:


* if the feature is narrow, one can simply observe as usual. In post-processing, the narrow feature can be flagged and the frequency gap interpolated by values of nearby channels, or by fitting a polynomial across the bandpass.


[[File:OPT-config1.png|200px|thumb|right|OPT - Instrument Configuration: Baseband Settings]]
* for wider contaminating lines, an option is to observe the bandpass at slightly offset frequencies and transfer the bandpass to the target frequency. If a common solution is obtained from two, symmetric offsets at higher and lower frequencies, the solution can be improved. Depending on the choice of offsets and also on the position in the receiver frequency range the error can vary. For 4 MHz offsets close to the HI rest frequency of 1.42GHz, the error is in the per cent range. A guide for CASA is described on [http://casaguides.nrao.edu/index.php?title=Combining_Bandpasses this CASAguides wiki page].


<!-- JUERGEN - some of this should be incorporated into the QuickStart Guide -->


[[File:OPT-config2.png|200px|thumb|right|OPT - Instrument Configuration: Subband Settings]]
=== Phase/Complex Gain Calibration ===


* '''Subband Setup''': Depending on the baseband setup, the '''Subband Configuration''' panel sports tabs for each baseband. Under each tab one can now select the individual subbands. Up to 64 subbands are available (16 for regular observing): click ''Add subband'' to create a subband setting and select the frequency range from the "Sky Range" drop-down menu. The ''Offset Freq from Center'' shows the placement of the subband with respect to the baseband center. For small bandwidths, the drop-down menu is not available as there are too many choices and the placement needs to be entered by hand in the ''Offset Freq from Center'' box. In teh current OSRO/RSRO interface, the subbands are not independently tuneable yet (but this feature will be available for the Ausgust 2012 deadline) and the subbands will snap on a frequency grid defined by the subband bandwidth. Now select the number of polarization products and the number of channels will be displayed in the ''Spectral Points'' box. The ''comments box'' can be used to describe the setup, e.g. by entering the transitions that should afll in that subband. Those entries are not used anywhere outside OPT '''(??? rest frequencies for OSRO ???)'''. The ''delete'' button removes the subband if is is no longer required, and ''Bulk Edit'' is used for bulk editing of many subbands (see the OPT guide on this feature). '''Note: if you chose subbands with different bandwidths during OSRO/RSRO, contact NRAO staff as the script currently need manual editing after OPT submission'''
The complex gain (phase/gain) calibration is the same for a spectral line observation as for any other observation. Ideally one should use the same correlator setup for the gain calibrator and the science target. For weak calibrators, however, it is possible to use wider bandwidths for the phase calibrator and then transfer the phases to the source. However, there will be a phase offset between them. The phase offset between the narrow and wide subbands can be determined by observing a strong source (e.g. the bandpass calibrator) and applied in post-processing from the complex gain calibrator to the target sources. A similar method can be used if the complex gain calibrator is observed at a slightly different frequency, e.g. to avoid a contaminating line feature such as Galactic HI.


* If not all subbands are used, one can use the remaining baseline boards to obtain a higher spectral resolution for those in the Subband Configuration panel. Select a higher number in the ''BL.BPS'' drop-down panel for baselineboard stacking. During commissioning, we recommend to use 2,4,8, etc. BlBs here but in principle any of the options in the drop down menu should work.


* Recirculation: only shared risk
=== Correlator Resources Setup ===


'''For the data taken early 2013 (August 2012 deadline), we will provide a specific new OPT Instrument Configuration layout for the regular and shared risk observing modes. The description below is for the current OSRO and RSRO modes. ''' <!-- JUERGEN - Not sure if you need to explain the current OPT resources, since this guide is designed for the Aug 2012 deadline.  Just a thought -->


Correctly specifying the WIDAR setup is essential for spectral line observations.  In the OPT, click on the ''Instrument Configuration'' tab. The most advanced setting is currently the RSRO setting (''File -> Create New -> RSRO Configuration''). This opens a page for the frequency setup as shown in the figures. Note that you may only have access to the OSRO configuration utility, depending on your history of VLA observations. The OSRO setup is a more restricted version of the RSRO setup.


==== Doppler Setting ====
* Enter a name and select the receiver in the top panel. This will adjust the available frequency range described below the drop down menu. The 1dB and 3dB ranges describe the roll-off behavior of the receiver sensitivity at the receiver band edges. If the frequency to be observed is close to the edge frequency of a receiver, one may check if the next higher or lower frequency receiver is more suitable.


[[File:Doppler.png|200px|thumb|right|OPT - Doppler Setting]]
* ''Baseband Tuning'': Select the position of the basebands. For the ''8bit samplers'', the central frequencies for 2 basebands are to be provided, each with a width of 1GHz. The center frequencies will go into the A0C0 box for frequency 1 and into the B0D0 box for frequency 2. For ''3bit samplers'', 4 basebands are available, each 2 GHz wide. Here, the center frequencies need to placed into the A1C1, A2C2, B1D1, and B2D2 boxes. A1C1 and A2C2 cannot be more than 4GHz apart, and the same restrictions apply to B1D1 and B2D2. Note that additional frequency restrictions may apply and the OPT/ICT will issue a clear warning or error message for those cases. The graphical panel above the input boxes shows the position of the two or four basebands. They are displayed as pairs to accommodate the R and L polarization inputs that may later be converted into single, dual, or full polarization products.


Doppler setting will calculate the sky frequency of your observation, based on the time of the observation, the velocity, position and rest frequency of a source and line. Contradictory to to Doppler tracking, Doppler setting calculates this once for each baseband at the start of the observation (execution of the SB) and the sky frequency will stay fixed for the entire run of the SB. Every subsequent run of the SB will perform a recalculation of the sky frequency. Doppler setting is currently only supported in OSRO mode (and will be available for the August 2012 deadline), RSRO observers need to calculate their own sky frequency (which is usually not too difficult given that the movement of the earth shifts the line by <math>\pm</math>30 km/s over a year, <math>\pm</math>0.5 km/s over a day - a velocity range that can under almost any circumstances be accommodated for by the wide bandwidths of WIDAR).
* ''Integration Time:'' this defines how data is dumped from the correlator backend into data files. A larger integration time will reduce the data volume. In addition, larger values can be chosen to take advantage of recirculation (only shared risk). On the other hand, time smearing effects, RFI excision, or time resolution may demand smaller integration times. It is important though, to not exceed the maximum data rate of 60MB/s (15MB/s for regular observing) and the integration time parameter is a good way to stay below this threshold for observations that demand large number of spectral channels.


To use Doppler Setting, first select ''Rest'' in the baseband frequency setup section of the OPT/Instrument Configuration Tool. All velocities will then be calculated against the center baseband frequency which may or may not be the rest frequency of your spectral line. Supply the reference position (a source from a catalog can be chosen, typically this will be your target source) and the velocity with their frames in the ''Doppler Setting'' section in the OPT/ICT. Doppler Setting can only be applied for each baseband and all subbands will shift by the same frequency amount, offset by the the subband tunings.
* The total data rates are displayed in the ''Configuration Summary''. The same panel also shows the number of baseline boards that are used in the setup. Remember that a maximum of 64 baseline boards are available and make sure that the data rate limits are not exceeded.


==== Bandpass Setup ====


All observations with the VLA - even those with the goal of observing continuum - require bandpass calibration. A bandpass calibrator should be bright enough, or observed long enough, so that the bandpass calibration does not significantly contribute to the noise in the image. This implies that, for a bandpass calibrator with flux density S<sub>cal</sub> observed for a time t<sub>cal</sub> and a science target with flux density S<sub>obj</sub> observed for a time t<sub>obj</sub>, <math> S_{cal} \sqrt{t_{cal}}</math> should be greater than <math>S_{obj} \sqrt{t_{obj}}</math>. How many times greater will be determined by one's science goals and the practicalities of the observations, but <math> S_{cal} \sqrt{t_{cal}}</math> should be greater by at least a factor of two. For extremely narrow channels or very weak bandpass calibrators, those typical flux requirements can lead to extremely large integration times. As an alternative one may then chose to interpolate in frequency, or to fit a polynomial across all channels in post-processing (''bandtype=BPOLY'' in CASA's [http://casa.nrao.edu/docs/TaskRef/bandpass-task.html bandpass task].
[[File:OPT-config1.png|200px|thumb|right|OPT - Instrument Configuration: Baseband Settings]]




The bandpass calibrator should be a point source or have a well-known model. At low frequencies, the absolute flux density calibrators (3C48, 3C147, or 3C286) are quite strong and in many cases can double as bandpass calibrators. At high frequencies, however, these sources have only moderate flux densities of ~0.5-3 Jy, translating into a potentially noisy bandpass solution. A different, stronger bandpass calibrator should then be observed.
[[File:OPT-config2.png|200px|thumb|right|OPT - Instrument Configuration: Subband Settings]]


The stability of bandpasses as a function of time is of concern for high-dynamic-range spectral work. We have found that most antennas show bandpasses that are stable to a few (~2-4) parts in a thousand over a period of several (~4-8) hours.  
* '''Subband Setup''': Depending on the baseband setup, the '''Subband Configuration''' panel sports tabs for each baseband. Under each tab one can now select the individual subbands. Up to 64 subbands are available (16 for regular observing): click ''Add subband'' to create a subband setting and select the frequency range from the "Sky Range" drop-down menu. The ''Offset Freq from Center'' shows the placement of the subband with respect to the baseband center. For small bandwidths, the drop-down menu is not available as there are too many choices and the placement needs to be entered by hand in the ''Offset Freq from Center'' box. In the current OSRO/RSRO interface, the subbands are not independently tunable yet (but this feature will be available for the August 2012 deadline) and the subbands will snap on a frequency grid defined by the subband bandwidth. Now select the number of polarization products and the number of channels will be displayed in the ''Spectral Points'' box. The ''comments box'' can be used to describe the setup, e.g. by entering the transitions that should fall in that subband. Those entries are currently not used anywhere outside OPT. The ''delete'' button removes the subband if is no longer required, and ''Bulk Edit'' is used for bulk editing of many subbands (see the OPT guide on this feature). '''Note: if you chose subbands with different bandwidths during OSRO/RSRO, contact NRAO staff as these scripts currently need manual editing after OPT submission.'''


Dramatic jumps in the bandpass structure (of order a few parts in a hundred) can occur at attenuator changes. The observer can track down such attenuator changes in their data using the switched power information; the On - Off power ('PDIF' in AIPS) will show a clear discontinuity. For this reason, it behooves the spectral line observer to observe a bandpass calibrator at least twice during their observations. Multiple observations will provide a check that all is well on most antennas and a mechanism for identifying any "problem" antennas. However, we do not expect that interpolating in time between consecutive bandpass solutions will bear much fruit for the observer. The low-level variations observed on some antennas tend to not be smooth functions of time and will likely not be corrected with interpolation.
* If not all subbands are used, one can use the remaining baseline boards to obtain a higher spectral resolution for those in the Subband Configuration panel. Select a higher number in the ''BL.BPS'' drop-down panel for baselineboard stacking. During commissioning, we recommend to use 2,4,8, etc. BlBs here but in principle any of the options in the drop down menu should work.


If there is only one observation of the bandpass calibrator, the observer should be careful to minimize the number of shadowed antennas, as an antenna without a bandpass determined for it will essentially be flagged in the data for the rest of the observation.
* Recirculation: only shared risk


We have shown that one can transfer the bandpass from a wide subband onto a narrow subband if the wide bandpass frequency range covers that narrow one. This may be good to the per cent level. But we advise to use that option only when absolutely necessary.
<!-- JUERGEN - maybe consider giving only a few pointers, and saying that the instructions are in the quick start guide. I'm not sure that you need specific instructions in this guide. -->


A complication can occur when the frequency range of the bandpass is contaminated by other spectral features such as rfi lines or Galactic HI  in absorption or emission. There are two basic options to accommodate that situation:
==== Using Doppler Setting ====


* if the feature is narrow, one can simply observe as usual. In post-processing, the narrow feature can be flagged and the frequency gap interpolated by values of nearby channels, or by fitting a polynomial across the bandpass.
[[File:Doppler.png|200px|thumb|right|OPT - Doppler Setting]]


* for wider contaminating lines, an option is to observe the bandpass at slightly offset frequencies and transfer the bandpass to the target frequency. If a common solution is obtained from two, symmetric offsets at higher and lower frequencies, the solution can be improved. Depending on the choice of offsets and also on the position in the receiver frequency range the error can vary. For 4 MHz offsets close to the HI rest frequency of 1.42GHz, the error is in the per cent range. A guide for CASA is described on this on [http://casaguides.nrao.edu/index.php?title=Combining_Bandpasses this CASAguides wiki].
Doppler setting will calculate the sky frequency of your observation based on the time of the observation, the source velocity, position and line rest frequency. In contrast to Doppler tracking, Doppler setting calculates this once '''for each baseband''' at the start of the observation (execution time of the SB) and the sky frequency will stay fixed for the entire run of the SB. Every subsequent run of the SB will perform a recalculation of the sky frequency. Doppler setting is currently only supported in OSRO mode (and will be available for the August 2012 deadline), RSRO observers need to calculate their own sky frequency (which is usually not too difficult given that the movement of the earth shifts the line by <math>\pm</math>30 km/s over a year, <math>\pm</math>0.5 km/s over a day - a velocity range that can under almost any circumstances be accommodated for by the wide bandwidths of WIDAR). <!-- JUERGEN - most of this is said above.  Not sure if you want it to be said twice. -->


==== Phase/Complex Gain Calibration ====
To use Doppler Setting, first select ''Rest'' in the baseband frequency setup section of the OPT/Instrument Configuration Tool. All velocities will then be calculated against the center baseband frequency which may or may not be the rest frequency of your spectral line. Supply the reference position (a source from a catalog can be chosen, typically this will be your target source) and the velocity with their frames in the ''Doppler Setting'' section in the OPT/ICT. Doppler Setting can only be applied for each baseband and all subbands will shift by the same frequency amount, offset by the the subband tunings.
 
The complex gain (phase/gain) calibration is the same for a spectral line observation as for any other observation. Ideally one should stick with the correlator setup for the target and the complex gain calibrator. For weak calibrators, however, it is possible to use wider bandwidths for the phase calibrator and then transfer the phases to the source. However, there will be a phase offset between them. The phase offset between the narrow and wide subbands can be determined by observing a strong source (e.g. the bandpass calibrator) and applied in post-processing from the complex gain calibrator to the target sources. A similar method can be used if the complex gain calibrator is observed at a slightly different frequency, e.g. to avoid a contaminating line feature such as Galactic HI.
 
==== Continuum Subtraction ====
 
The continuum of an observation can be either subtracted in the ''uv'' plane ([http://casa.nrao.edu/docs/TaskRef/uvcontsub-task.html ''uvcontsub''] in CASA) or in the image domain ([http://casa.nrao.edu/docs/TaskRef/imcontsub-task.html ''imcontsub'']). In most cases, the uv-domain is preferred but for high dynamic range imaging, or imaging close or beyond the edge of the FWHM of the primary beam, it may be better to subtract the continuum using line-free channels in the image cube, or to Fourier transform the com=ntinuum image and subtract these data from the visibilities ([http://casa.nrao.edu/docs/TaskRef/uvsub-task.html ''uvsub'']). All methods have in common that the continuum can only be determined accurately when enough line-free channels are observed that are necessary to derive a good model for the continuum. If the continuum sources exhibit significant spectral slope or even curvature, it is advisable to go to even larger bandwidth covering more line-free frequency space and to sample the continuum with many channels to ensure that a reasonable higher-order fit can be derived and subtracted from the line+continuum data.
 
 
 
==== High Dynamic Range Imaging ====
 
For very strong and narrow spectral features (typically thousands of Jy strong sources), one may see the Gibbs phenomenon (ringing) and Hanning smoothing may need to be applied (see above). This needs to be done in post-processing. The effect, however, can be reduced by using a higher spectral resolution such that the ringing effects beat against each other, effectively reduce the zig-zag pattern that appears on alternating, neighboring channels when the peak is within a single spectral channel.
 
 
<!--
= Spectral Line Observing =
 
''' everything below this section may be obsolete'''
 
 
== Current->Revised OSS Guidelines ==
 
* An Overview of the VLA
(last paragraph)
The VLA correlator will be extremely powerful and flexible. Details of the correlator configurations being offered for VLA early science during the period Sep 2011 - Dec 012 (a full D→A configuration cycle) are described in Correlator Configurations. It is important to realise that the VLA correlator is fundamentally a spectral line correlator. The days of separate “continuum” and “spectral line” modes of the VLA correlator are over, and all observations with the VLA will be “spectral line.” This has implications for how observations are set up, and users who may be used to continuum observing with the pre-upgrade VLA are strongly advised to consult Correlator Configurations.
 
* Limitations on Imaging Performance
(Sidelobes from Strong Sources)
An extension of the previous section is to very strong sources located anywhere in the sky, such as the Sun (especially when a flare is active), or when observing with a few tens of degrees of the very strong sources Cygnus A and Casseopeia A. Image degradation is especially notable at lower frequencies, shorter configurations, and when using narrow-bandwidth observations (especially in spectral line work) where chromatic aberration cannot be utilized to reduce the disturbances. In general, the only relief is to include the disturbing sources in the imaging, or to observe when these objects are not in the viewable hemisphere.
 
* Correlator Configurations
 
All observations with the VLA correlator should be treated as traditional VLA spectral line observations, in that they will require observation of a bandpass calibrator. They may also require observation of a delay calibrator. Users should contact NRAO staff for advice on setting up observations with the VLA correlator.


== Post-Processing Guidelines ==
== Post-Processing Guidelines ==


please see our [http://casaguides.nrao.edu/index.php?title=EVLA_Tutorials extensive VLA turoials] on the [http://casaguides.nrao.edu/ CASAguides wiki] for examples of how to process VLA spectra line data.
please see our [http://casaguides.nrao.edu/index.php?title=EVLA_Tutorials extensive VLA turoials] on the [http://casaguides.nrao.edu/ CASAguides wiki] for examples of how to process VLA spectra line data.
-->

Latest revision as of 14:05, 2 July 2012

The new Karl G. Jansky Very Large Array (VLA) has extremely powerful and versatile spectral capabilities. The final specifications include up to 4 million channels that can be distributed in up to 64 subbands with a spectral resolution from 2MHz down to the the Hz regime. This guide is intended to clarify the spectral line capabilities of the VLA, with a focus on capabilities offered for the August 2012 VLA proposal deadline, and enable users to plan, prepare, and process spectral observations.


Introducing VLA Spectral Line Observing

The newly available wide bandwidths of the VLA allow users to observe up to 8GHz of bandwidth at a time. All observations with the upgraded VLA are inherently spectral observations, including those intended for continuum science. The VLA's improved sensitivity and wide bandwidths greatly enhance the VLA's functionality for spectral line purposes, enabling simultaneous imaging of multiple spectral lines. The WIDAR correlator is extremely flexible and can act as up to 64 independent correlators with different bandwidths, channel numbers, polarization products, and observing frequencies. The final VLA will be able to

  • deliver continuous spectral coverage of up to 8GHz
  • access 1GHz or 2GHz chunks in each receiver band (called basebands) and place multiple correlator subbands within them
  • place up to 64 independently tunable subbands within a baseband; these can be configured with different bandwidths, channel numbers, and polarization products
  • tune the baseband and subband frequencies according to the object's velocity with respect to the earth (Doppler Setting)
  • dynamically schedule observations to use the best weather conditions for high frequency, high scientific impact projects


The following capabilities are offered for standard observing in the August 2012 proposal submission deadline. Greater flexibility is available through "shared risk" observing, as discussed below and detailed in the VLA Observational Status Summary.

  • Maximum data rates of 20MB/s
  • Doppler setting
  • 8-bit samplers providing
    • two 1GHz basebands
    • up to 16 independently tunable subbands per baseband
    • independent number of polarization products in each subband
    • independent subband bandwidths ranging from 31.25kHz to 128MHz
    • independent number of channels in each subband; maximum number of channels distributed across all subbands and polarization products cannot exceed 16384 channels
  • 3-bit samplers providing
    • four 2GHz basebands with a total of 64 128MHz subbands. 2MHz resolution full polarization, 1MHz dual and 0.5MHz single polarization

Line Frequencies and Velocity Conventions

Line Rest Frequencies

Spectral line calalogues available online are useful for selecting targeted line rest frequencies. The recommended catalog for VLA and ALMA observing is Splatalogue which contains molecular line data from sources including the Lovas catalog, the JPL/NASA molecular database], the Cologne Database for Molecular Spectroscopy, as well as radio recombination lines.

Observing Frequency and Velocity Definitions

The frequency at which we must tune the correlator in order to observe a spectral line () is derived from the radial velocity of the source (v) and the rest frequency of the spectral line (). The relativistic velocity, or true radial velocity, is related to the observed and rest frequencies by . This equation is a bit cumbersome to use; in astronomy two different approximations are typically used:

  • Optical Velocity ( is the redshift of the source)
  • Radio Velocity


The radio and optical velocities are not identical. Particularly, and diverge for large velocities. Optical velocities are predominantly used for extragalactic and radio velocities for Galactic targets.

At high redshifts, it is advisable to place the zero point of the velocity frame into the source via

,

where the redshifted can now be used as the input frequency for the observations. This method will appropriately apply the redshift correction to the channel and line widths and the resulting velocities are also intrinsic to the source.

Velocity Frames

The earth rotates, revolves around the sun, rotates around the galaxy, moves within the Local Group, and shows motion against the cosmic microwave background. Any source velocity must therefore always be specified relative to a reference frame.

Various velocity rest frames are used in the literature. The following table lists their name, the motion that is corrected for, and the maximum amplitude of the velocity correction. Each rest frame correction is incremental to the preceding row:


Rest Frame NameRest FrameCorrect forMax amplitude [km/s]
TopocentricTelescopeNothing0
GeocentricEarth CenterEarth rotation0.5
Earth-Moon BarycentricEarth+Moon center of massMotion around Earth+Moon center of mass0.013
HeliocentricCenter of the SunEarth orbital motion30
BarycentricEarth+Sun center of massEarth+Sun center of mass0.012
Local Standard of Rest (LSR)Center of Mass of local starsSolar motion relative to nearby stars20
GalactocentricCenter of Milky WayMilky Way Rotation230
Local Group BarycentricLocal Group center of massMilky Way Motion100
VirgocentricCenter of the Local Virgo superclusterLocal Group motion300
Cosmic Microwave BackgroundCMBLocal Supercluster Motion600

The velocity frame should be chosen based on the science. For most observations, however, one of the following three reference frames is commonly used:

  • Topocentric is the velocity frame of the observatory (defining the sky frequency of the observations). Visibilities in a measurement set are typically stored in this velocity frame.
  • Local Standard of Rest is the native output of images in CASA. Note that there are two varieties of LSR: the kinematic LSR (LSRK) and the dynamic (LSRD) definitions for the kinematic and dynamic centers, respectively. In almost all cases LSRK is being used and the less precise name 'LSR' is usually used synonymously with more modern LSRK definition.
  • Barycentric is a commonly used frame that has virtually replaced the older heliocentric standard. Given the small difference between the barycentric and heliocentric frames, they were frequently used interchangeably.


A full list of CASA supported reference frames is provided in the CASA reference Manual and Cookbook and also on the casaguides.nrao.edu webpage

Doppler Correction

A telescope naturally operates at a fixed sky frequency (Topocentric velocity frame) which can be adjusted to account for the motion of the earth. A spectral line's observed frequency will shift during any observing campaign. Within a day, the rotation of the earth dominates and the line may shift up to 0.5km/s, depending on the position of the source on the sky (see above). Observing campaigns that span a year may have spectral lines that shift by up to 30km/s due to the earth's motion around the sun.

Note: As a rule of thumb, 1 MHz in frequency corresponds roughly to km/s for the line at a wavelength of in mm. E.g., at a wavelength of 7mm, 1MHz corresponds to about 7km/s in velocity, at 21cm 1MHz corresponds roughly to 210km/s.

Using this rule of thumb, a line may shift by up to 5MHz in Q-band and by up to 0.15MHz in L-band over the course of a year. This needs to be taken into account when setting up the observations. This issue can be handled in different ways:

  • use the same sky frequency for all observations, accommodating the line shift (maximum of 30km/s) by using a wide enough bandwidth to cover the line at any time in the observing campaign. The data is later regridded in CASA to a common LSRK or BARY velocity frame. The sky frequency of an observation can be computed with the Dopset tool for a given time. One may find the LST dates for an observation on the VLA Schedule Page.
  • calculate the sky frequency at the beginning of an observing block and keep this fixed for the duration of the scheduling block. This is called Doppler Setting and offered by OPT for each baseband (currently only in OSRO mode). The line shift is then reduced to the rotation of the earth (maximum amplitude 0.5km/s). This small shift is corrected in data processing.
  • change the sky frequency continuously to keep the line at the same position in the band. This method is called Doppler tracking and was standard for the pre-upgrade VLA. The new VLA does NOT support Doppler tracking. The WIDAR correlator offers enough bandwidth and spectral channels to cover any line shift and post-processing regridding needs. In addition, a non-variable sky frequency may also deliver a more robust calibration and overall system stability.

The regridding of the spectrum can be completed during data processing in CASA, either directly during imaging in the task clean, or alternatively with the task cvel. The regridding works well when the spectral features are sampled with at least 4 channels.

The WIDAR Correlator

Basebands

Let's start with the basics: A signal from the telescope enters WIDAR, is split into its left and right hand circular polarizations, and passes through analog filters that define the basebands. Basebands set the spectral range that can be accessed by subbands, and they come in baseband pairs to cover L and R polarizations. Each baseband pair can be set to one baseband sky frequency. Basebands are the most fundamental spectral ranges delivered from the samplers and digitizers to the correlator. With 8-bit sampling, the samplers deliver two independently tuneable baseband pairs (dubbed A0/C0 and B0/D0) with 1 GHz bandwidth each. 3-bit sampling provides four baseband pairs (A1/C1, A2/C2, B1/D1, B2/D2), each of them 2GHz wide.

Baseband with WIDAR subbands

Baseband Tuning Restrictions

The following restrictions apply to baseband tuning:

  • With 8-bit sampling in Ka band, only one baseband can be below 32GHz and that must be B0/D0
  • 3-bit samplers can only be used in C-band or above, where the instantaneous frequency width of the receiver is larger than 2GHz.
  • The 3-bit A1C1 baseband frequency can have a separation of max. 4GHz from the A2D2 baseband. B1D1 and B2D2 have the same restriction of 4GHz maximum separation

Fixed 128MHz Subbands and 128MHz "Suckouts"

After filtering through the basebands, the signal enters the correlator and is split into fixed, 128MHz wide subbands. They are placed adjacently to cover the full width of the basebands. Narrow subbands can be arranged within these fixed 128MHz subbands. Because each fixed 128MHz subband has a filter shape with soft corners, the sensitivity of the VLA drops to about half its maximum value between any two fixed 128MHz subbands. These frequency ranges are called "128 MHz Suckouts". There are two primary options for dealing with the suckouts:

1. Try to set the baseband frequency such that targeted lines do not fall in the suckouts. We offer the spectral line setup tool "TUNE" that can be used to maximize the frequency separation between a number of spectral lines and the suckouts.


2. If it is not possible to obtain coverage of all of your lines using the above method, observe with two basebands shifted by 10-64MHz apart. This will ensure that at one baseband covers the suckouts of the other baseband with full sensitivity. An example is given in the figures to the right.


rms noise in a blank field as a function of frequency for one baseband consisting of 8 contiguous sub-bands. Note the increased rms noise at the subband edges.


rms noise in a blank field as a function of frequency for two basebands consisting of 8 contiguous sub-bands, where the basebands are separated by one-half of the subband width. Wherever signal in one baseband is compromised by edge effects, data from the other subband are substituted.

Correlator Resources and Subband Placement

Correlator baselineboards (BlBs, also named "BL.BPS" for "baselineboard pairs" in the OPT) are independent hardware units that are allocated to narrow subbands. WIDAR has 64 baselineboard pairs. As a result, WIDAR supports a maximum of 64 subband pairs (again, pairs to cover the two polarizations), but the number of subbands depends on the subband setups as described below. For the same number of channels, single (RR or LL), dual (RR & LL), and full (RR, LL, RL, and LR) polarization products require 1, 2, and 4 times the correlator baselineboards respectively. Similarly, doubling the number of channels in a subband doubles the number of correlator resources used.

Narrow Subbands with the 8-bit sampler

Narrow subbands define the frequency ranges in which the spectrum is measured. Narrow subbands with bandwidths between 31.25kHz and 128MHz can be arranged to obtain desired frequency coverage and spectral resolution within the baseband. Each narrow subband needs to be entirely within a fixed 128MHz subband, as narrow subbands cannot cross a 128MHz suckout frequency. The baseband center frequency can be shifted and subband bandwidths and frequencies must be arranged to avoid the suckouts. This implies that the 128MHz fixed subbands cannot be moved as they would fall on a suckout at any frequency offset from a 128MHz "raster" within the baseband. All subbands less than 128MHz in width, however, and can be independently tuned as long as they do not cross a suckout. Furthermore, all subbands can be set up with different bandwidths, frequency resolutions, channel numbers, and polarization products.

Standard Subbands

Standard subbands allocate a single baselineboard pair to a each subband. Standard subbands contain 64 channels when full polarization (RR,LL, RL & LR) products are required, 128 channels in dual polarization mode (RR & LL), and 256 channels for single polarization (RR or LL). Options for full and dual polarization subbands, with frequency and velocity resolutions, are shown in the following tables. For the August 2012 deadline, we offer up to 16 subbands per baseband for normal observations, and up to 64 for shared risk observations.

  • Full polarization
Sub-band BW (MHz) Number of channels/poln product Channel width (kHz) Channel width (km/s at 1 GHz) Total velocity coverage per sub-band (km/s at 1 GHz)
128 64 2000 600/ν(GHz) 38,400/ν(GHz)
64 64 1000 300 19,200
32 64 500 150 9,600
16 64 250 75 4,800
8 64 125 37.5 2,400
4 64 62.5 19 1,200
2 64 31.25 9.4 600
1 64 15.625 4.7 300
0.5 64 7.813 2.3 150
0.25 64 3.906 1.2 75
0.125 64 1.953 0.59 37.5
0.0625 64 0.977 0.29 18.75
0.03125 64 0.488 0.15 9.375
  • Dual Polarization
Sub-band BW (MHz) Number of channels/poln product Channel width (kHz) Channel width (km/s at 1 GHz) Total velocity coverage (km/s at 1 GHz)
128 128 1000 300/ν(GHz) 38,400/ν(GHz)
64 128 500 150 19,200
32 128 250 75 9,600
16 128 125 37.5 4,800
8 128 62.5 19 2,400
4 128 31.25 9.4 1,200
2 128 15.625 4.7 600
1 128 7.813 2.3 300
0.5 128 3.906 1.2 150
0.25 128 1.953 0.59 75
0.125 128 0.977 0.29 37.5
0.0625 128 0.488 0.15 18.75
0.03125 128 0.244 0.073 9.375

Baselineboard Stacking

In order to obtain a larger number of channels per subband, a method called "baselineboard stacking" can be used. Baselineboard stacking allows a larger number of baseline boards to be allocated to a single subband, increasing the number of channels within the subband. To double the number of channels in a subband, simply double the number of baselineboards allocated to the subband. Thus each additional BlB for a subband adds another 64 channels in full and 128 channels in dual, and 256 channels in single polarization modes.

As an example, the full 2GHz bandwidth of the 8-bit samplers can be covered by 16 128MHz standard narrow subbands, each with 128 channels in dual polarization, as shown in the table above. The 16 subbands, however, only require 16 BlBs and another 48 are available for baselineboard stacking. One can thus use 4 BlBs for each of the subbands, quadrupling the number of channels from 128 to 512, which reduces the channel widths from 1MHz to 0.25 MHz over the full 2 GHz frequency range. This method works for any subband bandwidth.

Baselineboard stacking can be very useful for spectral line work, as it allows for wider bandwidths for each subband while maintaining frequency resolution. With baselineboard stacking, you can use fewer subbands to cover a set frequency range, thereby minimizing the number of filter edges and resulting sensitivity dropoff. Baselineboard stacking is offered for the August 2012 deadline. For shared risk observations, however, we recommend that observers do not use all 64 baselineboards to allow for observing in the event that one or two baselineboards are not working on any given day.


Recirculation

"Recirculation" is a second method for obtaining more spectral channels in a given subband. Recirculation uses the fact that the correlator has more computing capability when the data is averaged in time. The standard correlator dump time is 1s. If this is doubled to 2s, WIDAR can produce twice as many channels as listed in the tables above. 4s would allow 4 times the number of channels. Recirculation is only available for shared risk observations for the August 2012 proposal deadline.

Narrow Subbands with the 3-bit sampler

As the 3-bit samplers are still under commissioning, the August 2012 proposal deadline offers only a single observing setup with 3-bit sampling. This setup includes full coverage over all four baseband pairs (using 64 128MHz subband) 2MHz resolution for full polarization, 1MHz dual, and 0.5MHz for single polarization.

Data Rate Limits

Baselineboard stacking, recirculation, and time resolution can add up to an extremely high data rate in the correlator. The VLA currently supports data rates of up to 20MB/s for regular and more for shared risk observing, see the Observational Status Summary for details. The OPT instrument configuration calculates data rates based on the spectral line setup and the data rate maxima should not be exceeded for any observational setup.

Tips for Planning, Setup, and Processing of VLA Spectral Line Observations

Reminder: The following capabilities are offered for regular observing for the August 1 2012 proposal deadline. Consult the VLA Observational Status Summary for additional options available through shared risk observing:


  • Maximum data rates of 20MB/s
  • Doppler setting
  • 8-bit samplers providing
    • two 1GHz basebands
    • up to 16 independently tunable subbands per baseband
    • independent number of polarization products in each subband
    • independent subband bandwidths ranging from 31.25kHz to 128MHz
    • independent number of channels in each subband; maximum number of channels distributed across all subbands and polarization products cannot exceed 16384 channels
  • 3-bit samplers providing
    • four 2GHz basebands with a total of 64 128MHz subbands. 2MHz resolution full polarization, 1MHz dual and 0.5MHz single polarization


Considerations for Planning Subband Bandwidths and Resolution

Bandwidths required for UV continuum subtraction

When determining the bandwidths needed in your subbands, it is important to observe enough line-free channels on each side of the spectral line to allow for good continuum subtraction. It is possible to interpolate the continuum levels from one subband to another, but it is usually a better solution to derive the continuum level in each subband separately. The number of line free channels ideally equals or exceeds the number of channels that cover the line. When the line free channels are distributed equally on both sides of the spectral line, a low order polynomial (polynomials of order 1 appear to be good models for most cases) usually provides a good fit. Whenever higher order polynomials are needed, e.g. for a continuum source with a significant spectral curvature over the subband(s), the number of line-free channels should be increased.

Channel Widths for High Dynamic Range Imaging: Dealing with the Gibbs Phenomenon

For very sharp spectral or lag features, the spectrum can prominently display a ringing effect known as the Gibbs phenomenon, a sinc function that zig-zags on alternating channels. If this is apparent in the data, smoothing adjacent channels will reduce or even eliminate the effect. Hanning smoothing is the most effective method, which uses a triangular smoothing kernel with the central channel weighed by 0.5 and the two adjacent channels by 0.25. After Hanning smoothing, however, the channels are not independent anymore and one can eliminate every other channel without losing signal to noise.

In the pre-upgrade VLA days, the correlator design had a realtively short, truncated lag spectrum, which could result in prominent Gibbs ringing. To avoid this effect, Hanning smoothing was frequently applied online during the observations. With the new WIDAR capabilities of the upgraded VLA, however, ringing is very rare and only observed for extremely strong maser or RFI sources. Consequently, the VLA does not support online Hanning smoothing anymore; if required, Hanning smoothing can be applied during post-processing (e.g. with the CASA task hanningsmooth.). The Gibbs effect can also be reduced by using higher spectral resolution that covers the spectral feature with several channels. In that case, the ringing effects from the individual channels beat against each other, effectively reducing the zig-zag pattern that appears on alternating, neighboring channels when the peak is within a single spectral channel.

Sensitivity/Exposure Time Calculation

The VLA Exposure Calculator

After planning your general setup and sensitivity needs, the required on-source observing time is best calculated with the VLA Exposure Calculator. This JAVA tool (see screenshot) allows one to input the required rms noise and bandwidth limits and outputs the required time on source given a frequency, weather, weighting scheme, and number of polarization products. The input Bandwidth should correspond to the frequency resolution that is required to perform the science. This may or may not be the width of individual spectral channels. Overheads need to be added according to our VLA frequently asked questions webpage. We refer to the low frequency guide and high frequency guide for further advice on how to set up the observations, depending on the receiver band to be used.

The Proposal Submission Tool (PST)

The Proposal Submission Tool (PST, accessible at my.nrao.edu ) is used to submit proposals, including the scientific justification, abstract, and authors, as well as planned target sources, observing session lengths, and correlator setups. In order to ensure that planned correlator setups comply with the capabilities offered for the August 2012 deadline, the Proposal Submission Tool (PST) includes a spectral setup tool.


Snapshot of the 8-bit sampler, 2x1GHz PST setup tool

In the example to the right, 9 subbands were chosen in Ka band with 8-bit sampling. Four of the subbands are in the first and five in the second baseband. The setup features different subband bandwidths, polarization products and uses baselineboard stacking (up to 16 baseline board pairs per subband pair are used for a couple of subbands). A total of 49 baseline boards are used for this configuration.


Snapshot of the 3-bit sampler, wideband mode PST setup tool

Wide-band mode (3-bit sampler, up to 8GHz bandwidth) can be used for spectral line observing as well. The channel width is fixed to a resolution of 2MHz for full, 1MHz for dual and 0.5MHz for single polarization products. No narrow subbands can be chosen.

Setting up a Spectral Observation using the Observation Preparation Tool (OPT)

The Observation Preparation Tool (OPT) is the web-based interface to create scheduling blocks (SBs) for time awarded on the VLA. An SB is the observing program used for a single observing run. This consists of at least a few startup scans, a pointing reference, a bandpass calibration, a flux calibration, gain calibration and target observations. In the OPT, you specify your sources, scan lengths and order, and correlator setups. A full project may consist of several SBs. To access the OPT, go to my.nrao.edu and click on the Obs Prep tab, followed by Login to the Observation Preparation Tool. Instructions for using the OPT and for selecting appropriate calibrators are provided in the OPT QuickStart Guide, and a comprehensive user's manual and up to date information on the OPT are available at OPT webpage. As such, this guide provides only brief notes on the bandpass and gain calibrators and then focuses on the task of setting up correlator resources.

Bandpass Setup

All observations with the VLA - even those with the goal of observing continuum - require bandpass calibration. When scheduling the bandpass calibration scans within an SB, the observer should be careful to minimize the number of shadowed antennas, as an antenna without a bandpass determined for it will essentially be flagged in the data for the rest of the observation. A bandpass calibrator should be bright enough, or observed long enough, so that the bandpass calibration does not significantly contribute to the noise in the image. This implies that, for a bandpass calibrator with flux density Scal observed for a time tcal and a science target with flux density Sobj observed for a time tobj, should be greater than . How many times greater will be determined by one's science goals and the practicalities of the observations, but should be greater by at least a factor of two. For extremely narrow channels or very weak bandpass calibrators, those typical flux requirements can lead to extremely large integration times. As an alternative one may then chose to reduce the integration time and interpolate in frequency, or to fit a polynomial across all channels in post-processing (bandtype=BPOLY in CASA's bandpass task.

The bandpass calibrator should be a point source or have a well-known model. At low frequencies, the absolute flux density calibrators (3C48, 3C147, or 3C286) are quite strong and can often double as bandpass calibrators. At high frequencies (Ku, K, Ka, Q), however, these sources have only moderate flux densities of ~0.5-3 Jy, translating into a potentially noisy bandpass solution. A different, stronger bandpass calibrator should then be observed. Naturally, all of the above depends on the channel widths and for wide channels the standard flux calibrators may be sufficient even at higher frequencies. In turn, extremely narrow channels may require stronger bandpass calibrators at the low frequency end. Additionally, We have shown that one can transfer the bandpass from a wide subband onto a narrow subband if the wide bandpass frequency range covers that narrow one. This may be good to a level of a few per cent, but we advise to use that option only when absolutely necessary.

The stability of bandpasses as a function of time is of concern for high-dynamic-range spectral work. We have found that most antennas show bandpasses that are stable to a few (~2-4) parts in a thousand over a period of several (~4-8) hours. This should be sufficient for most scientific goals but the bandpasses can be observed several times during an observation for extreme calibration accuracy requirements.

A complication can occur when the frequency range of the bandpass is contaminated by other spectral features such as RFI lines or Galactic HI in absorption or emission. There are two basic options to accommodate that situation:

  • if the feature is narrow, one can simply observe as usual. In post-processing, the narrow feature can be flagged and the frequency gap interpolated by values of nearby channels, or by fitting a polynomial across the bandpass.
  • for wider contaminating lines, an option is to observe the bandpass at slightly offset frequencies and transfer the bandpass to the target frequency. If a common solution is obtained from two, symmetric offsets at higher and lower frequencies, the solution can be improved. Depending on the choice of offsets and also on the position in the receiver frequency range the error can vary. For 4 MHz offsets close to the HI rest frequency of 1.42GHz, the error is in the per cent range. A guide for CASA is described on this CASAguides wiki page.


Phase/Complex Gain Calibration

The complex gain (phase/gain) calibration is the same for a spectral line observation as for any other observation. Ideally one should use the same correlator setup for the gain calibrator and the science target. For weak calibrators, however, it is possible to use wider bandwidths for the phase calibrator and then transfer the phases to the source. However, there will be a phase offset between them. The phase offset between the narrow and wide subbands can be determined by observing a strong source (e.g. the bandpass calibrator) and applied in post-processing from the complex gain calibrator to the target sources. A similar method can be used if the complex gain calibrator is observed at a slightly different frequency, e.g. to avoid a contaminating line feature such as Galactic HI.


Correlator Resources Setup

For the data taken early 2013 (August 2012 deadline), we will provide a specific new OPT Instrument Configuration layout for the regular and shared risk observing modes. The description below is for the current OSRO and RSRO modes.

Correctly specifying the WIDAR setup is essential for spectral line observations. In the OPT, click on the Instrument Configuration tab. The most advanced setting is currently the RSRO setting (File -> Create New -> RSRO Configuration). This opens a page for the frequency setup as shown in the figures. Note that you may only have access to the OSRO configuration utility, depending on your history of VLA observations. The OSRO setup is a more restricted version of the RSRO setup.

  • Enter a name and select the receiver in the top panel. This will adjust the available frequency range described below the drop down menu. The 1dB and 3dB ranges describe the roll-off behavior of the receiver sensitivity at the receiver band edges. If the frequency to be observed is close to the edge frequency of a receiver, one may check if the next higher or lower frequency receiver is more suitable.
  • Baseband Tuning: Select the position of the basebands. For the 8bit samplers, the central frequencies for 2 basebands are to be provided, each with a width of 1GHz. The center frequencies will go into the A0C0 box for frequency 1 and into the B0D0 box for frequency 2. For 3bit samplers, 4 basebands are available, each 2 GHz wide. Here, the center frequencies need to placed into the A1C1, A2C2, B1D1, and B2D2 boxes. A1C1 and A2C2 cannot be more than 4GHz apart, and the same restrictions apply to B1D1 and B2D2. Note that additional frequency restrictions may apply and the OPT/ICT will issue a clear warning or error message for those cases. The graphical panel above the input boxes shows the position of the two or four basebands. They are displayed as pairs to accommodate the R and L polarization inputs that may later be converted into single, dual, or full polarization products.
  • Integration Time: this defines how data is dumped from the correlator backend into data files. A larger integration time will reduce the data volume. In addition, larger values can be chosen to take advantage of recirculation (only shared risk). On the other hand, time smearing effects, RFI excision, or time resolution may demand smaller integration times. It is important though, to not exceed the maximum data rate of 60MB/s (15MB/s for regular observing) and the integration time parameter is a good way to stay below this threshold for observations that demand large number of spectral channels.
  • The total data rates are displayed in the Configuration Summary. The same panel also shows the number of baseline boards that are used in the setup. Remember that a maximum of 64 baseline boards are available and make sure that the data rate limits are not exceeded.


OPT - Instrument Configuration: Baseband Settings


OPT - Instrument Configuration: Subband Settings
  • Subband Setup: Depending on the baseband setup, the Subband Configuration panel sports tabs for each baseband. Under each tab one can now select the individual subbands. Up to 64 subbands are available (16 for regular observing): click Add subband to create a subband setting and select the frequency range from the "Sky Range" drop-down menu. The Offset Freq from Center shows the placement of the subband with respect to the baseband center. For small bandwidths, the drop-down menu is not available as there are too many choices and the placement needs to be entered by hand in the Offset Freq from Center box. In the current OSRO/RSRO interface, the subbands are not independently tunable yet (but this feature will be available for the August 2012 deadline) and the subbands will snap on a frequency grid defined by the subband bandwidth. Now select the number of polarization products and the number of channels will be displayed in the Spectral Points box. The comments box can be used to describe the setup, e.g. by entering the transitions that should fall in that subband. Those entries are currently not used anywhere outside OPT. The delete button removes the subband if is no longer required, and Bulk Edit is used for bulk editing of many subbands (see the OPT guide on this feature). Note: if you chose subbands with different bandwidths during OSRO/RSRO, contact NRAO staff as these scripts currently need manual editing after OPT submission.
  • If not all subbands are used, one can use the remaining baseline boards to obtain a higher spectral resolution for those in the Subband Configuration panel. Select a higher number in the BL.BPS drop-down panel for baselineboard stacking. During commissioning, we recommend to use 2,4,8, etc. BlBs here but in principle any of the options in the drop down menu should work.
  • Recirculation: only shared risk


Using Doppler Setting

OPT - Doppler Setting

Doppler setting will calculate the sky frequency of your observation based on the time of the observation, the source velocity, position and line rest frequency. In contrast to Doppler tracking, Doppler setting calculates this once for each baseband at the start of the observation (execution time of the SB) and the sky frequency will stay fixed for the entire run of the SB. Every subsequent run of the SB will perform a recalculation of the sky frequency. Doppler setting is currently only supported in OSRO mode (and will be available for the August 2012 deadline), RSRO observers need to calculate their own sky frequency (which is usually not too difficult given that the movement of the earth shifts the line by 30 km/s over a year, 0.5 km/s over a day - a velocity range that can under almost any circumstances be accommodated for by the wide bandwidths of WIDAR).

To use Doppler Setting, first select Rest in the baseband frequency setup section of the OPT/Instrument Configuration Tool. All velocities will then be calculated against the center baseband frequency which may or may not be the rest frequency of your spectral line. Supply the reference position (a source from a catalog can be chosen, typically this will be your target source) and the velocity with their frames in the Doppler Setting section in the OPT/ICT. Doppler Setting can only be applied for each baseband and all subbands will shift by the same frequency amount, offset by the the subband tunings.

Post-Processing Guidelines

please see our extensive VLA turoials on the CASAguides wiki for examples of how to process VLA spectra line data.

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