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| = Introduction = | | = EVLA Observational Status = |
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| == Purpose of Document ==
| | * [[Observational Status Summary]]: Describes expected OSRO capabilities for the next D→A configuration cycle, September 2011 through January 2013, for use in preparing proposals for the August 1, 2011, proposal deadline, and following (updated 29 Jun 2011). |
| | | * [[Observational Status Summary - Current]]: Describes current OSRO capabilities through the end of the A configuration, September 2011, for use by observers and rapid response science proposers (updated 15 May 2010). |
| 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/.
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| 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.
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| == What is the Expanded Very Large Array? ==
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| 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.
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| == VLA to EVLA Transition ==
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| 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.
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| {| border="1"
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| |+ '''Table 1: Overall EVLA Performance Goals'''
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| !Parameter
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| !VLA
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| !EVLA
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| !Factor
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| |-
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| | Continuum Sensitivity (1-σ, 9 hr) || 10 μJy || 1 μJy || 10
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| |-
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| | Maximum BW in each polarization || 0.1 GHz || 8 GHz || 80
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| |-
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| | Number of frequency channels at max. BW || 16 || 16,384 || 1024
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| |-
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| | Maximum number of freq. channels || 512 || 4,194,304 || 8192
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| | Coarsest frequency resolution || 50 MHz || 2 MHz || 25
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| | Finest frequency resolution || 381 Hz || 0.12 Hz || 3180
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| |-
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| | Number of full-polarization sub-correlators || 2 || 64 || 32
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| |-
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| | Log (Frequency Coverage over 1–50 GHz) || 22% || 100% || 5
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| |-
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| |}
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| Note: The "Factor" gives the factor by which the EVLA parameter will be an improvement over the equivalent VLA parameter.
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| {| border="1"
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| |+ '''Table 2: EVLA Major Milestones'''
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| !Milestone
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| !Target Date
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| | Installation of EVLA correlator subset for early science || 2010 Q1
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| | Shared Risk Observing begins || 2010 Q1
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| | Full EVLA correlator installation || 2010 Q2
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| | Last antenna retrofitted || 2010 Q2
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| | Last receiver installed || 2012 Q3
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| |-
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| |}
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| = An Overview of the EVLA =
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| 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.
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| 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.
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| 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.
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| 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.
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| {| border="1"
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| |+ '''Table 3: Predicted EVLA Configuration Schedule for 2010-2011'''
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| ! Year
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| ! Feb-May
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| ! Jun-Sep
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| ! Oct-Jan
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| | 2010 || D || C || B
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| | 2011 || A || D || C
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| |}
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| 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.
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| 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.
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| 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.
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| 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.
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| = EVLA Early Science =
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| 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.
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| 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.
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| == Expected Capabilities: Antennas ==
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| 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.
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| == Expected Capabilities: Receivers ==
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| 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.
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| 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.
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| = Performance of the EVLA =
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| 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.
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| == Resolution ==
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| 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.
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| 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.
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| 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.
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| == Sensitivity ==
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| The theoretical thermal noise expected for an image using natural weighting of the visibility
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| data is given by:
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| <math>\alpha^2+\beta^2=1</math>
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