Introduction

Purpose of Document

This document summarizes the current instrumental status of the Expanded Very Large Array (EVLA). It is intended as a ready reference for those contemplating use of the EVLA for their astronomical research. The information is in summary form – those requiring greater detail should consult the EVLA’s staff members, listed in Section 8, or refer to the manuals and documentation listed in Section 7. Most of the information contained here, and much more, is available on the Web, and can be accessed through the EVLA home page information for astronomers, at http://science.nrao.edu/evla/, and the VLA information for astronomers, http://www.vla.nrao.edu/astro/. These pages will shortly be combined into a single page for the EVLA, and the links in this document will be updated accordingly. A companion document for the VLBA is also available from http://science.nrao.edu/vlba/.

The EVLA is a large and complex modern instrument. It cannot be treated as a “black box,” and some familiarity with the principles and practices of its operation is necessary before efficient use can be made of it. Although the NRAO strives to make using the EVLA as simple as possible, users must be aware that proper selection of observing mode and calibration technique is often crucial to the success of an observing program. Inexperienced and first-time users are especially encouraged to enlist the assistance of an experienced colleague or NRAO staff member for advice on, or direct participation in, an observing pro- gram. Refer to Section 5.14 for details. The EVLA will be an extremely flexible instrument, and we are always interested in imaginative and innovative ways of using it.

What is the Expanded Very Large Array?

The EVLA is the product of a program to modernize the electronics of the Very Large Array (VLA) in order to improve several key observational parameters by an order of magnitude or more. Some of the details of the EVLA Project may be found on the web, at http://www.aoc.nrao.edu/evla/. The EVLA is funded jointly by the US National Science Foundation (NSF), the Canadian National Research Council, and the CONACyT funding agency in Mexico. Total funding is approximately $94 million in Year 2006 dollars, including $59 million in new NSF funding, $16 million in redistributed effort from the NRAO Operations budget, $17 million for the correlator from Canada, and $2 million from Mexico. The EVLA project will be completed on time and on budget in 2012, 11 years after it began. Its key observational goals are (1) complete frequency coverage from 1 to 50 GHz; (2) continuum sensitivity improvement by up to an order of magnitude (nearly two orders of magnitude in speed) by increasing the bandwidth from the VLA’s 100 MHz per polarization to 8 GHz per polarization; and (3) implementation of a new correlator that can process the large bandwidth with a minimum of 16,384 spectral channels per baseline. A comparison of some of the EVLA performance parameters with those of the VLA is provided in Table 1. The remaining major milestones for the EVLA are shown in Table 2.

VLA to EVLA Transition

The year 2010 will be extremely exciting for the EVLA. The correlator that has been the heart of the VLA for three decades was decommissioned on 11 January, 2010. The VLA will be shut down to outside users until early March 2010, during which time hardware willbe transferred from the old correlator to the EVLA correlator and observing modes will be commissioned in preparation for EVLA early science. At the same time the direction of the configuration cycles will also change, from A→B→C→D→A to D→C→B→A→D, in order to facilitate the EVLA correlator commissioning and to limit initial EVLA data rates. When the telescope returns to general use it will be the EVLA.

Table 1: Overall EVLA Performance Goals

Parameter	VLA	EVLA	Factor
Continuum Sensitivity (1-σ, 9 hr)	10 μJy	1 μJy	10
Maximum BW in each polarization	0.1 GHz	8 GHz	80
Number of frequency channels at max. BW	16	16,384	1024
Maximum number of freq. channels	512	4,194,304	8192
Coarsest frequency resolution	50 MHz	2 MHz	25
Finest frequency resolution	381 Hz	0.12 Hz	3180
Number of full-polarization sub-correlators	2	64	32
Log (Frequency Coverage over 1–50 GHz)	22%	100%	5

Note: The "Factor" gives the factor by which the EVLA parameter will be an improvement over the equivalent VLA parameter.

Table 2: EVLA Major Milestones

Milestone	Target Date
Installation of EVLA correlator subset for early science	2010 Q1
Shared Risk Observing begins	2010 Q1
Full EVLA correlator installation	2010 Q2
Last antenna retrofitted	2010 Q2
Last receiver installed	2012 Q3

An Overview of the EVLA

The EVLA is a 27-element interferometric array, arranged along the arms of an upside- down “Y”, which will produce images of the radio sky at a wide range of frequencies and resolutions. It is located at an elevation of 2100 meters on the Plains of San Agustin in southwestern New Mexico, and is managed from the Pete V. Domenici Science Operations Center (DSOC) in Socorro, New Mexico.

The basic data produced by the EVLA are the visibilities, or measures of the spatial coherence function, formed by correlation of signals from the array’s elements. The most common mode of operation will use these data, suitably calibrated, to form images of the radio sky as a function of sky position and frequency. Another mode of observing (commonly called phased array) will allow operation of the array as a single element through coherent summation of the individual antenna signals. This mode will most commonly be used for VLBI observing and for observations of rapidly varying objects, such as pulsars. However, it will not be available initially.

The EVLA can vary its resolution over a range exceeding a factor of ∼ 50 through movement of its component antennas. There are four basic arrangements, called configura- tions, whose scales vary by the ratios 1 : 3.28 : 10.8 : 35.5 from smallest to largest. These configurations are denoted D, C, B, and A respectively. In addition, there are 3 “hybrid” configurations labelled DnC, CnB, and BnA, in which the North arm antennas are de- ployed in the next larger configuration than the SE and SW arm antennas. These hybrid configurations are especially well suited for observations of sources south of δ = −15◦ or north of δ = +75◦, for which the foreshortening of the longer North arm results in a more circular point spread function.

Traditionally, the VLA completed one cycle through all four configurations in ap- proximately a 16 month period. However, this period will likely change in early 2010 to accommodate commissioning of the EVLA correlator and the onset of EVLA early science. The present best estimate for the EVLA configuration schedule in 2010 and 2011 is presented in Table 3, but prospective users should consult the web page http://science.nrao.edu/evla/proposing/configpropdeadlines.shtml or re- cent NRAO and AAS newsletters for up-to-date schedules and associated proposal deadlines. Refer to Section 5.1 for information on how to submit an observing proposal.

Table 3: Predicted EVLA Configuration Schedule for 2010-2011

Year	Feb-May	Jun-Sep	Oct-Jan
2010	D	C	B
2011	A	D	C

Observing projects on the EVLA will vary in duration from as short as 1/2 hour to as long as several weeks. Most observing runs have durations of a few to 24 hours, with only one, or perhaps a few, target sources. However, since the EVLA is a two-dimensional array, images can be made with data durations of less than one minute. This mode, commonly called snapshot mode, is well suited to surveys of relatively strong, isolated objects. See Section 4.15 for details.

All EVLA antennas will eventually be outfitted with eight receivers providing continuous frequency coverage from 1 to 50 GHz. These receivers will cover 1–2 GHz, 2–4 GHz, 4–8 GHz, 8–12 GHz, 12–18 GHz, 18–26.5 GHz, 26.5–40 GHz, and 40–50 GHz. These bands are commonly referred to as L, S, C, X, Ku, K, Ka, and Q bands, respectively. See Section 3.2 for more details about the availability of new bands.

The VLA’s original P-band (300 – 340 MHz) receivers are incompatible with the EVLA’s wideband electronics, so there is at present no P-band observing capability. The NRAO, in cooperation with NRL, is now developing a wideband receiver system which will provide improved P-band performance. Tests of this new system will be carried out during 2010, but there is not yet an implementation date. This new receiver system will also replace the existing 74-MHz (4-band) receivers. It is unlikely that the 74-MHz capability will be available in 2010.

The EVLA correlator will be extremely powerful and flexible. Details of the correlator configurations to be offered for EVLA early science are described in Section 4.13. It is important to realise that the EVLA 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 EVLA will be “spectral line.” This has implications for how observations are set up, and users who may be used to continuum observing with the VLA are strongly advised to consult Section 4.13.

EVLA Early Science

EVLA early science will be provided by two programs for outside users and one for EVLA commissioning staff. All early science programs will be peer-reviewed. In keeping with a primary construction project goal, the EVLA will continue to be used for science throughout the commissioning of the telescope into full operations in 2013. Observing during this period will thus involve an element of risk associated with the large stepwise increases in throughput bandwidth that will be offered to the community at the start of each new array configuration cycle in 2010, 2011, and 2012.

The Open Shared Risk Observing (OSRO) program will provide early science capabilities to the general user community. These capabilities will initially provide a maximum 256 MHz bandwidth that will increase to 2 GHz in mid-2011 and to 8 GHz in 2012. The Resident Shared Risk Observing (RSRO) program will provide these capabilities, and other more powerful ones, much sooner to users who can reside in Socorro and help with the EVLA commissioning efforts. These same enhanced capabilities will also be made available to EVLA commissioning staff via the EVLA Commissioning Staff Observing (ECSO) program.

Expected Capabilities: Antennas

Retrofitted EVLA antennas are being returned to the array to be used as part of normal operations at the rate of approximately one antenna every two months. At the beginning of EVLA early science there will be 26 antennas in the array. The remaining two VLA antennas will be decommissioned while their retrofits are completed; they can not be used in conjunction with the EVLA correlator until they have been converted to the EVLA antenna design. All conversions will be completed by mid-2010.

Expected Capabilities: Receivers

All retrofitted EVLA antennas are outfitted with either EVLA or “interim” L, EVLA or “interim” C, VLA X, EVLA K, and EVLA Q-band receivers. (Interim receivers are EVLA receivers with narrowband VLA polarizers. All interim receivers will be converted to full EVLA capabilities by the end of 2012. The polarization purity and sensitivity of the interim receivers typically is good only over the traditional VLA tuning range.) As of January 2010, 17 of the EVLA antennas are also outfitted with EVLA Ka-band receivers, and 6 EVLA antennas have S-band receivers. Figure 1 shows the expected rate of antenna retrofits and installation of the final EVLA receiver systems throughout the EVLA construction project. The 8-GHz maximum bandwidth availability depends on the implementation of the fast 3-bit samplers (the “8 GHz BW” line in Figure 1). Prior to this, the maximum available bandwidth will be 2 GHz per polarization.

Figure 1: EVLA Receiver Deployment Plan. Above is a plot of the availability of the final EVLA receivers from late 2009 until the end of the EVLA Construction Project in 2012. Only final EVLA receivers are shown. Interim receivers with reduced frequency coverage or polarization purity are available at some bands (see Table 4). Approximate installation dates for the full 8 GHz bandwidth per polarization also are shown.

Figure 1 does not tell the entire story of frequency availability for observing with the EVLA, however, since there are interim or VLA receivers at L, C, and X-bands that can be used in the absence of the final EVLA receivers. Table 4 gives a prediction of the new frequency capabilities that we expect in June 2010, along with the expected “total” numbers of receivers for a given band, including VLA-style and/or interim receivers. New receiver bands will be offered for general use when the performance of at least five antennas has been verified by EVLA commissioning staff.

Table 4: Tuning Ranges of EVLA Bands

Band	Range	Receiver availability, June 2010
	GHz	Final EVLA systems	Total EVLA+VLA/interim
20 cm (L)	1.0-2.0	9	26
13 cm (S)	2.0-4.0	10	10
6 cm (C)	4.0-8.0	22	26
3 cm (X)	8.0-12.0	0	27
2 cm (Ku)	12.0-18.0	5	5
1.3 cm (K)	18.0-26.5	27	27
1 cm (Ka)	26.5-40.0	25	25
0.7 cm (Q)	40.0-50.0	27	27

Note: The rightmost column gives the total numbers of receivers expected to be available for a given band, including all VLA-style and/or interim receivers. Only the old narrow-band VLA receivers are available in the 8-8.8 GHz band until closer to the end of the EVLA construction project (see Figure 1).

Performance of the EVLA

This section contains details of the EVLA’s resolution, expected sensitivity, tuning range, dynamic range, pointing accuracy, and modes of operation. Detailed discussions of most of the observing limitations are found elsewhere. In particular, see References 1 and 2, listed in Section 7.

Resolution

The EVLA’s resolution is generally diffraction-limited, and thus is set by the array config- uration and frequency of observation. It is important to be aware that a synthesis array is “blind” to structures on angular scales both smaller and larger than the range of fringe spacings given by the antenna distribution. For the former limitation, the EVLA acts like any single antenna – structures smaller than the diffraction limit (θ ∼ λ/D) are broadened to the resolution of the antenna. The latter limitation is unique to interferometers; it means that structures on angular scales significantly larger than the fringe spacing formed by the shortest baseline are not measured. No subsequent processing can fully recover this missing information, which can only be obtained by observing in a smaller array configuration, by using the mosaicing method, or by utilizing data from an instrument (such as a large single antenna or an array comprising smaller antennas) which provides this information.

Table 5 summarizes the relevant information. This table shows the maximum and minimum antenna separations, the approximate synthesized beam size (full width at half- power), and the scale at which severe attenuation of large scale structure occurs.

A project with the goal of doubling the longest baseline available in the A configuration by establishing a real-time fiber optic link between the VLA and the VLBA antenna at Pie Town was established in the late 1990s, and used through 2005. This link is no longer operational; there is a goal (unfunded, at present) of implementing a new digital Pie Town link after the EVLA construction project has been completed.

Sensitivity

The theoretical thermal noise expected for an image using natural weighting of the visibility data is given by:

${\displaystyle \alpha ^{2}+\beta ^{2}=1}$

where:

– SEFD is the “system equivalent flux density” (Jy), defined as the flux density of a radio source that doubles the system temperature. Lower values of the SEF D indicate more sensitive performance. For the EVLA’s 25-meter paraboloids, the SEFD is given by the equation SEFD = 5.62Tsys/ηA, where Tsys is the total system temperature (receiver plus antenna plus sky), and ηA is the antenna aperture efficiency in the given band.

– ηc is the correlator efficiency (at least 0.92 for the EVLA).

– npol is the number of polarization products included in the image; npol = 2 for images in Stokes I, Q, U, or V, and npol = 1 for images in ‘RCP’ or ‘LCP’.

– N is the number of antennas.

– tint is the total on-source integration time in seconds.

– ∆ν is the bandwidth in Hz.

Figure 2 shows the SEFD as a function of frequency for those bands currently installed on EVLA antennas, and include the contribution to Tsys from atmospheric emission at the zenith. Table 6 gives the SEFD at some fiducial EVLA frequencies. At X-band, where the VLA-style receivers are still in use, the SEFD is approximately 310 Jy. Because EVLA testing with WIDAR has been used with a limited subset of antennas, it has not yet been possible to test whether equation 1 holds for an image made using the full array. However, it is expected that this will be the case, and equation 1 should be used for estimating required integration times.

Table 5: Configuration Properties

Configuration	A	B	C	D
Bmax (km1)	36.4	11.1	3.4	1.03
Bmin (km1)	0.68	0.21	0.0355	0.035
	Synthesized Beamwidth thetaHPBW(arcsec)1,2,3
1.5 GHz (L)	1.3	4.3	14	46
3.0 GHz (S)6	0.65	2.1	7.0	23
6.0 GHz (C)	0.33	1.0	3.5	12
8.5 GHz (X)7	0.23	0.73	2.5	8.1
15 GHz (Ku)6	0.13	0.42	1.4	4.6
22 GHz (K)	0.089	0.28	0.95	3.1
33 GHz (Ka)6	0.059	0.19	0.63	2.1
45 GHz (Q)	0.043	0.14	0.47	1.5
	Largest Angular Scale thetaLAS(arcsec)1,4
1.5 GHz (L)	36	120	970	970
3.0 GHz (S)6	18	58	490	490
6.0 GHz (C)	8.9	29	240	240
8.5 GHz (X)7	6.3	20	170	170
15 GHz (Ku)6	3.6	12	97	97
22 GHz (K)	2.4	7.9	66	66
33 GHz (Ka)6	1.6	5.3	44	44
45 GHz (Q)	1.2	3.9	32	32

These estimates of the synthesized beamwidth are for a uniformly weighted, untapered map produced from a full 12 hour synthesis observation of a source which passes near the zenith.

Footnotes:

1. B_max is the maximum antenna separation, B_min is the minimum antenna separation, theta_HPBW is the synthesized beam width (FWHM), and theta_LAS is the largest scale structure "visible" to the array.

2. The listed resolutions are appropriate for sources with declinations between −15 and 75 degrees. For sources outside this range, the extended north arm hybrid configurations (BnA, CnB, DnC) should be used, and will provide resolutions similar to the smaller configuration of the hybrid, except for declinations south of −30. No double-extended north arm hybrid configuration (e.g., CnA, or DnB) is provided.

3. The approximate resolution for a naturally weighted map is about 1.5 times the numbers listed for theta_HPBW. The values for snapshots are about 1.3 times the listed values.

4. The largest angular scale structure is that which can be imaged reasonably well in full synthesis observations. For single snapshot observations the quoted numbers should be divided by two.

5. The standard C configuration has been replaced by a slightly modified one, formerly known as CS, wherein an antenna from the middle of the north arm has been moved to the central pad “N1”. This results in improved imaging for extended objects, but will degrade snapshot performance. Although the minimum spacing is the same as in D configuration, the surface brightness sensitivity to extended structure is considerably inferior to that of the D configuration (but considerably better than standard C configuration).

6. The S, Ku, and Ka bands do not yet have a full complement of antennas, so the exact values will depend on the rate of antenna outfitting and the placement of individual antennas in the various configurations.

7. At X-band the default VLA frequency of 8.5 GHz has been assumed, since there are no EVLA 8–12 GHz receivers available yet and the VLA-style receivers will probably be used through the end of 2010.

Table 6: SEFDs and D-Configuration Confusion Limits

Frequency	SEFD	RMS confusion level
	(Jy)	in D config (\mu Jy/beam)
1.5 GHz (L)	420	89
3.0 GHz (S)	370	14
6.0 GHz (C)	310	2.3
22 GHz (K)	560	...
33 GHz (Ka)	600	...
45 GHz (Q)	1400	...

Note: SEFDs at K, Ka, and Q bands include contributions from Earth's atmosphere, and were determined under good conditions. At X-and, wehre the VLA-style receivers are still in use, the SEFD is approximately 310 Jy. The confusion limits in C configuration are approximately a factor of 10 less than those listed above.

Note that the theoretical rms noise calculated using equation 1 is the best limit possible. There are several factors that will tend to increase the noise compared with theoretical:

• For the more commonly-used “robust” weighting scheme, intermediate between pure natural and pure uniform weightings (available in the AIPS task IMAGR and CASA task clean), typical parameters will result in the sensitivity being a factor of about 1.2 worse than the listed values.

• Confusion. There are two types of confusion: (i) that due to confusing sources within the synthesized beam, which affects low resolution observations the most. Table 6 shows the confusion noise in D configuration (see Condon 2002, ASP Conf. 278, 155), which should be added in quadrature to the thermal noise in estimating expected sensitivities. The confusion limits in C configuration are approximately a factor of 10 less than those in Table 6; (ii) confusion from the sidelobes of uncleaned sources lying outside the image, often from sources in the sidelobes of the primary beam. This primarily affects low frequency observations.

• Weather. The sky and ground temperature contributions to the total system temper- ature increase with decreasing elevation. This effect is very strong at high frequencies, but is relatively unimportant at the other bands. The extra noise comes directly from atmospheric emission, primarily from water vapor at K-band, and from water vapor and the broad wings of the strong 60 GHz O2 transitions at Q-band.

In general, the zenith atmospheric opacity to microwave radiation is very low – typi- cally less than 0.01 at L, C and X-bands, 0.05 to 0.2 at K-band, and 0.05 to 0.1 at the lower half of Q-band, rising to 0.3 by 49 GHz. The opacity at K-band displays strong variations with time of day and season, primarily due to the 22 GHz water vapor line. Conditions are best at night, and in the winter. Q-band opacity, dominated by atmospheric O2, is considerably less variable.

Observers should remember that clouds, especially clouds with large water droplets (read, thunderstorms!), can add appreciable noise to the system temperature. Sig- nificant increases in system temperature can, in the worst conditions, be seen at frequencies as low as 5 GHz.

Tipping scans can be used for deriving the zenith opacity during an observation. In general, tipping scans should only be needed if the calibrator used to set the flux density scale is observed at a significantly different elevation than the range of elevations over which the phase calibrator and target source are observed. When the flux density calibrator observations are within the elevation range spanned by the science observing, elevation dependent effects (including both atmospheric opacity and antenna gain dependencies) can be accounted for by fitting an elevation-dependent gain term. See the following item.

• Antenna elevation-dependent gains. The antenna figure degrades at low elevations, leading to diminished forward gain at the shorter wavelengths. The gain-elevation effect is negligible at frequencies below 8 GHz. The antenna gains can be determined by direct measurement of the relative system gain using the AIPS task ELINT on data from a strong calibrator which has been observed over a wide range of elevation. If this is not possible, care should be taken to observe a primary flux calibrator at the same elevation as the target.

The AIPS task INDXR applies standard elevation-dependent gains and an estimated opacity in CL table version 1. The CASA calibration tasks (e.g. gaincal, bandpass) also use the standard gain curves. Note that currently the gain coefficients used in AIPS and CASA are for VLA antennas; the elevation-dependent gains of the EVLA antennas have yet to be fully characterized.

• Pointing. The SEFD quoted assumes good pointing. Under calm nighttime condi- tions, the antenna blind pointing is about 10 arcsec rms. The pointing accuracy in daytime is a little worse, due to the effects of solar heating of the antenna structures. Moderate winds have a very strong effect on both pointing and antenna figure. The maximum wind speed recommended for high frequency observing is 15 mph (7 m/s). Wind speeds near the stow limit (45 mph) will have a similar negative effect at 8 and 15 GHz.

To achieve better pointing, “referenced pointing” is recommended, where a nearby calibrator is observed in interferometer pointing mode every hour or so. The local pointing corrections thus measured can then be applied to subsequent target observations. This reduces rms pointing errors to as little as 2 – 3 arcseconds if the reference source is within about 10 degrees (in azimuth and elevation) of the target source, and the source elevation is less than 70 degrees.

Use of referenced pointing is highly recommended for all K, Ka, and Q-band observa- tions, and for lower frequency observations of objects whose total extent is a significant fraction of the antenna primary beam. It is usually recommended that the referenced pointing measurement be made at 8 GHz (X-band), regardless of what band your target observing is at, since X-band is the most sensitive, and the closest calibrator is likely to be weak. Proximity of the reference calibrator to the target source is of paramount importance; ideally the pointing sources should precede the target by 20 or 30 minutes in time. The calibrator should have at least 0.5 Jy flux density at X-band and be unresolved on all baselines to ensure an accurate solution.

To aid EVLA proposers there is an exposure tool calculator on-line at http://science.nrao.edu/evla/tools/exposure/evlaExpoCalc.jnlp that provides a graphical user interface to these equations. The beam-averaged brightness temperature measured by a given array depends on the synthesized beam, and is related to the flux density per beam by:

EVLA Frequency Bands and Tunability

For OSRO observations each receiver can tune to two different frequencies from the same wavelength band. Right-hand circular (RCP) and left-hand circular (LCP) polarizations are received for both frequencies. Each of these four data streams currently follows the VLA nomenclature, and are known as IF (for “Intermediate Frequency” channel) “A”, “B”, “C”, and “D”. IFs A and B receive RCP, IFs C and D receive LCP. IFs A and C are always at the same frequency, as are IFs B and D (but the IFs A/C frequency is usually different from the B/D frequency). We normally refer to these two independent data streams as “IF pairs.”

The tuning ranges, along with default frequencies for continuum applications, are given in Table 7 below. The EVLA X and Ku bands are not yet available, although the old narrow-band VLA X-band receivers may still be used.