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Guide to Proposing for the VLA

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1. Obtaining Observing Time on the VLA

How to Propose

Observing time on the VLA is available to all researchers, regardless of nationality or location of institution. There are no quotas or reserved blocks of time. The allocation of observing time on the VLA is based upon the submission of a VLA Observing Proposal using the online Proposal Submission Tool (PST) available via the NRAO Interactive Service web page, at https://my.nrao.edu/. The online tool permits the detailed construction of a cover sheet specifying the requested observations, using a set of online forms, and uploading of a PDF format scientific justification to accompany the cover information. More specific info on using the online tool can be found in the NRAO User Portal manual. VLA-specific details, including capabilities offered for the next proposal deadline, can be found in the Offered VLA Capabilities chapter of the current VLA Observational Status Summary (OSS) document. For further information on observing at the VLA, including information on radio frequency interference (RFI), 8/3 bit Attenuation and Setups, and pre-submission checklists for Scheduling Blocks and instrument configurations, please see the Guide to Observing with the VLA.

It is also possible to obtain VLA observing time by proposing to NASA missions, under cooperation agreements established between NRAO and those missions. Such programs exist for the Chandra, Fermi, Swift and HST missions. Astronomers interested in those joint programs should consult the relevant mission proposal calls for more information.

Note that it is possible to search cover sheets of proposals previously approved for time using the Proposal Finder Tool (PFT).

Students

Students planning to use the VLA for their Ph.D. dissertation must submit a Plan of Dissertation Research of no more than 1000 words with their first proposal. This plan can be referred to in later proposals. At a minimum, the plan should contain a thesis time line and an estimate of the level of VLA resources needed. The plan provides some assurance against a dissertation being impaired by an adverse review of a proposal when the full scope of the thesis is not seen. The plan can be submitted via NRAO Interactive Services. Also see the Plan of Dissertation Research subsection of the NRAO User Portal manual for more details. Students are reminded to submit their plan comfortably in advance of the proposal deadline. New thesis plans must be in PDF format to enable science reviewers to easily access the plans. Students who have not yet graduated, but have active plans on file, should consider updating those plans to a PDF format if they are not already in that form.

Timeline

Time on the VLA is scheduled on a 6-month semester basis. Semester A observations typically take place February through July and have an August 1 proposal deadline in the previous year; Semester B observations, with deadline February 1, take place from August through January. If the deadline falls on a weekend it is extended to the next working day. The call for proposals, which typically goes out 3–4 weeks prior to the deadline, includes an overview of upcoming deadlines and VLA configurations.

For details on the evaluation of submitted proposals we refer to the NRAO Proposal Evaluation/Time Allocation page. Because of competition, even highly ranked proposals are not guaranteed to receive observing time. This is particularly true for proposals that concentrate on objects in the LST ranges occupied by popular targets such as the Galactic Center or Virgo. Daytime VLA observing will also continue to be limited by ongoing testing and maintenance activities; for more information, see the section on Scheduling Considerations in this document.

Director's Discretionary Time

It is also possible to propose for Director's Discretionary Time (DDT). DDT is reserved for Targets of Opportunity and for Exploratory Time. DDT proposals may be submitted at any time with the understanding that they should only request for the current and, maybe, the next upcoming array configuration. The DDT proposals must be submitted through the PST. DDT proposals are reviewed by Observatory scientific staff on the basis of the proposals' scientific merit, conflict status, LST pressure, and technical feasibility.

2. Array Configurations

Introduction

The VLA is reconfigurable and uses four principal array configurations, A through D. The A-configuration provides the longest baselines and thus the highest angular resolution for a given frequency, but yields very limited sensitivity to surface brightness. The D-configuration provides the shortest baselines, translating to a high surface brightness sensitivity at the cost of angular resolution. See the configuration schedule for details for each call for proposals and the highest angular resolution as function of frequency in the OSS. In general, as the baseline length expressed in wavelengths gets longer, the phase stability gets worse which impacts the observing strategy and observing overhead.

It is generally important to consider the following:

  • What angular resolution is required for the proposed science at the desired observing frequency?
  • For resolved sources, how does the desired angular resolution compare to the required surface brightness sensitivity?
  • For the array configuration that gives the desired angular resolution, how much of the flux density is actually in compact components, and will not be resolved out?

The Observational Status Summary section on Resolution, in conjunction with the Exposure Calculator, can help answer these questions.

Please note that low declination sources risk being subject to antenna shadowing at certain azimuths for the C and D configurations. These targets can still be observed. Observing at low declination implies, however, smaller windows of no-shadowing (one on either side of the north-south arm), which effectively makes the setup of the scheduling blocks harder or the observation less sensitive than expected.

Note on hybrid configurations: For very southern (and very northern) declinations, the VLA used to offer hybrid array configurations where the north arm was extended compared to the east and west arms. This provided a more circular synthesized beam at declinations south of −15 deg and north of 75 deg, at the cost of limited scheduling opportunities due to the short duration of the hybrids. Semester 2016A was the last semester to offer the hybrid configurations. See the following section for alternatives to the hybrids.

Alternatives to Hybrid Configurations

The approach required to substitute for the lack of hybrid VLA configurations depends on the science goal and observing mode of a proposal as described below. We assume projects that would have proposed to use the hybrid configurations are requesting them because they wish to observe sources at southern (<−15 deg) or northern (>75 deg) declinations.

Point Sources

Proposers should request the next largest principal configuration and ask for the same amount of observing time that would have been requested in a hybrid.

Extended Structure

Good surface brightness sensitivity is needed for projects aiming to image an extended structure. Assuming one is trying to match the surface brightness sensitivity of a hybrid configuration (rather than matching the surface brightness sensitivity to a particular science goal) we can use the density of visibilities, Nvis, as a function of uv-distance, as a measure of the sensitivity to different spatial scales. The sensitivity is then proportional to (Nvis)-1/2. The goal of matching the surface brightness sensitivity of a particular hybrid configuration is therefore one of matching the visibility density as a function of uv-distance through a combination of integration time in one or several principal configurations.

Figure 2.2.1 shows a graphical visualization useful for quantifying Nvis vs. uv-distance, for an example using ν=3GHz, δ=-25 deg, aiming to match the visibility density of the BnA hybrid for a snapshot observation. It demonstrates that to match the visibility density in the inner uv-plane of the BnA hybrid a combination of the same amount of integration time that would have been requested for the BnA hybrid (tint=tBnA) is needed in the A configuration, along with an additional 40% of the hybrid time (tint=0.4tBnA) in the B configuration (this combination is referred hereafter as A+0.4B). Alternatively, double the integration time in the A configuration (tint=2tBnA, hereafter 2A) can match the BnA visibility density at short uv spacings. A similar result is found for the CnB configuration.

Figure 2.2.2 shows how the additional time in the more compact configuration is a function of declination, and also the dependence on track length. A long (e.g., 5 or 6-hour) earth rotation synthesis is able to fill the uv-plane more effectively due to the decreased antenna shadowing when the target is away from the meridian compared with a snapshot or even a 2-hour synthesis centered on the meridian. BnA can be well reproduced by A+0.4B, and CnB can be reproduced by B+0.4C. For reproducing the visibility density of the DnC hybrid the shadowing on short baselines in the D configuration for δ<-25 deg is severe, and unless a long synthesis is possible it is more effective at these low declinations to double the hybrid time request and observe entirely in the C configuration.

Figure 2.2.3 presents the RMS noise as a function of the geometric mean FWHM of the synthesized beam derived for the same snapshot example in Figure 2.2.1 above, for a variety of uv-tapers and natural weighting. It demonstrates that A+0.4B matches well the surface brightness sensitivity of the BnA hybrid on spatial scales of at least 10 times the untapered, naturally weighted BnA synthesized beam of 2-arcsec, while 2A matches the RMS noise of the hybrid for spatial scales out to ~5 times the untapered, naturally weighted BnA synthesized beam.

In summary, then, the surface brightness sensitivity of a hybrid configuration can be reproduced by:
- either doubling the on-source integration time of the next largest principal configuration, or;
- combining 1.0 × (larger configuration) + 0.4 × (smaller configuration).
The choice of which to use depends on the science goal, declination, and observing mode, as described below. An exception is the DnC hybrid, for which shadowing at very low declinations makes the use of the D configuration very inefficient. To substitute for the DnC hybrid, proposers with targets at δ<-25 deg should always request double the amount of DnC observing time in C configuration.

  • Imaging extended structure with on-source integration times of around a minute or more:
  1. CnB/BnA substitute: proposers should combine 1.0 × (larger configuration) + 0.4 × (smaller configuration).
  • Note: if the field contains variable sources, but the science goal is imaging of extended structure, proposers should either request double the time in the next largest principal configuration (if this can be accommodated in a single scheduling block), or be prepared to model and subtract variable sources from individual datasets prior to combining, as needed.
  • DnC substitute: proposers with targets at δ=-25 deg or higher should combine C+0.4D, as for CnB and BnA; proposers with targets at δ<-25 deg should request double the amount of DnC time but in the C configuration.
    • Imaging extended structure with very short on source integration times (e.g., large mosaics): very short scans can result in large slewing overheads, so to optimize observing efficiency proposers should request double the on-source time that would have been requested in the associated hybrid, for the next largest principal configuration.

    For more details on how to optimize the science on the VLA without the hybrid configurations, we refer to the EVLA memo 193Also, proposers should direct any questions about which configuration they should use to the NRAO Helpdesk.

    3. Scheduling Considerations

    Sessions

    Scheduling priorities for the VLA are based on the use of sessions in the PST, which are the units of observing time considered by the Time Allocation Committee. Since the observing priority assigned to a session can strongly depend on how the proposer structures these sessions, we provide a set of guidelines to help you in defining your sessions.

    High Frequency

    High frequency observations are susceptible to atmospheric effects, such as air turbulence and water vapor. During the summer months, and especially during daytime, conditions appropriate for high frequency observing are limited. Other limitations may include needing night-time observing or observing during dry seasons for increased phase stability. With the stricter weather constraints on high frequency (> 15 GHz) observations, programs are in competition for less available time. Figure 3.2.1 illustrates a sample plot showing the fraction of available days, as a function of LST, for observing at various bands in April 2011.

    For more information on high frequency observations, please see the High Frequency Strategy section of the Guide to Observing with the VLA.

     

    Low Frequency

    Observations using the lower frequency bands of the VLA may be adversely affected by Radio Frequency Interference (RFI). There is extensive documentation on RFI at the VLA. Please see the Radio Frequency Interference section in the Guide to Observing. Furthermore, such observations may be impacted by solar activities that can cause disturbing ionospheric effects. For more details, see the Low Frequency Strategy and the Very Low Frequency Strategy sections of the Guide to Observing with the VLA.

     

    LST Range

    For a number of reasons, the observer might prefer night-time observing over daytime observing:

    • During the night, phase stability tends to be better, which is important for high-frequency observing, and there is no solar interference which could affect low-frequency observing.
    • There is less time available for observing during the day because of scheduled maintenance and testing on most weekdays, and a session with sources visible only during the day may end up with lower observing priority than when its sources are up during the night.

    Figure 3.2.2 shows the daytime LST range as a function of the day of the year, which can assist you in planning your observations. We note that for moving sources, such as planets, the situation becomes more complex as their LST range suitable for observing tends to vary during the course of the year.

     

    Moving Objects

    The VLA is able to observe moving objects (solar system bodies) in standard continuum modes as part of general observing. It is not currently possible to observe spectral lines in planets or comets, except in unusual circumstances (background source occultations, for instance), or as part of the Resident Shared Risk Observing (RSRO) program.

    Also be aware that during the configuration, moving objects may be observed at LST ranges that are considerably different between the beginning and ending date. Please indicate in the proposal when the optimum date ranges are, and what LST ranges they correspond to. It is beneficial to avoid the high-pressure LST ranges if the object can be observed at other times. For extended objects, also comment on the largest angular scales necessary in the imaging and what date ranges need to be avoided as the object would be too close/large.

    For more information on observing moving objects with the VLA, please see the Moving Objects section of the Guide to Observing at the VLA.

     

    Sun Avoidance

    Throughout the course of the year, the Sun moves across the sky with respect to background objects. Due to the NRAO's use of dynamic scheduling, a scheduling block may end up being observed when the Sun is close enough to have an adverse effect on the data with phase fluctuations and elevated system temperatures.

    Please see the section on Avoiding the Sun in the Guide to Observing. There is an online tool available to check and see how close your sources are to the Sun and Moon; here is the link to the VLA Sun & Moon Distance Check Tool.

    4. Frequency Bands and Samplers

    Frequency Bands

    All VLA antennas are outfitted with eight cryogenically cooled receivers providing continuous frequency coverage from 1 to 50 GHz. These receivers cover the frequency ranges of 1–2 GHz (L-band), 2–4 GHz (S-band), 4–8 GHz (C-band), 8–12 GHz (X-band), 12–18 GHz (Ku-band), 18–26.5 GHz (K-band), 26.5–40 GHz (Ka-band), and 40–50 GHz (Q-band). Additionally, all antennas of the VLA have receivers for lower frequencies, enabling observations at P-band (230–470 MHz). These low frequency receivers also work at 4-band (54–86 MHz), and new feeds have been deployed on fourteen VLA antennas to observe at this frequency range. This number is expected to rise to seventeen in early 2018.

    For more information on frequency bands and tuning, please see the VLA Frequency Bands and Tunability section in the VLA Observational Status Summary.

    Samplers

    The VLA is equipped with two different types of samplers: 8-bit with 1GHz bandwidth each and 3-bit with 2GHz bandwidth each.

    The 8-bit set consists of four 8-bit samplers arranged in two pairs, each pair providing 1024 MHz bandwidth in both polarizations. The two pairs are denoted A0/C0 and B0/D0. Taken together, the four samplers offer a maximum of 2048 MHz coverage with full polarization. The frequency spans sampled by the two pairs need not be adjacent. Some restrictions apply, depending on band (e.g., the Ka-band), as described in the section on Frequency Bands and Tunability.

    The 3-bit set consists of eight 3-bit samplers arranged as four pairs, each pair providing 2048 MHz bandwidth in both polarizations. Two of these pairs, denoted A1/C1 and A2/C2, cannot span more than 5000 MHz (lower edge of one to the higher edge of the other). The same limitation applies to the second pair, denoted B1/D1 and B2/D2. The tuning restrictions are described in the section on Frequency Bands and Tunability. Taken together, the eight 3-bit samplers offer a maximum of 8192 MHz coverage with full polarization.

    For more information on samplers and which set to use, please see the VLA Samplers section in the VLA Observational Status Summary.

    5. Correlator Setup

    The correlator configuration depends entirely on the science goal. For continuum science, choose the widest bandwidth per subband with coarse spectral resolution; typically the NRAO default settings (possibly retuned to alternative center frequencies) are sufficient. For spectral line, choose the subband bandwidth and the spectral resolution that best fit the scientific objectives of your project. Detailed information on the correlator is available in the WIDAR section of the VLA OSS.

    Issues that should be kept in mind are:

    • The widest (128 MHz) subbands in a baseband do not overlap. Additionally, a few channels may need to be flagged at either subband edge because of the higher noise due to filter roll-off. If the science goal requires a homogeneous sensitivity sampling over multiple 128 MHz subbands, we recommend tuning the second baseband at a frequency that is offset by a fraction of a subband width with respect to the first baseband. This, however, removes the possibility to place the second baseband freely in the receiver band to do other science. Also, anywhere from 8 MHz to up to 30 MHz at the edges of the basebands may be noisier, so you should not rely on a spectral line that would be close to a baseband edge. For more details, see the Subband 0 subsection of the Spectral Line section in the Guide to Observing with the VLA. 
    • It may be necessary to Hanning smooth your data in order to get rid of Gibbs ringing (for the theory behind this phenomenon see Gibbs phenomenon). Lower frequency bands (X and below) are prone to strong Radio Frequency Interference (RFI); flagging the RFI could be close to impossible unless you first Hanning smooth your data. This necessity should be taken into account when choosing the spectral resolution of your proposed observations, since the effective resolution will be lower than the original (pre-Hanning smoothed), even though the number of channels will stay the same. Note that the frequency resolution (FWHM) of un-tapered spectra is 1.2×Δν (where Δν is the channel spacing) and the resolution of Hanning-tapered spectra is 2.0×Δν .
    • For spectral line observations, given an expected line width, it is a good idea to select a spectral resolution that will allow for at least 4–5 channels across your line, or twice that many when Hanning smoothing. There are a number of tools available online to identify molecular line rest frequencies such as the Lovas Catalog and Splatalogue. The frequency range covered by ~10 to 50 GHz (X to Q-band) contains a large number of diagnostically interesting atomic and molecular transitions. For continuum data only, it is wise to check whether the chosen frequency range contains potentially strong spectral lines.
    • Any correlator configuration should stay within the allowed data rate limits for GO and SRO observing. This might become an issue with complex correlator setups using recirculation and/or short integration times. GOST will report the data rate for a specific correlator setup.

     

     

      6. General Observing Setup Tool (GOST)

      For DDT submissions after the Call for Proposals but before the approaching deadline only: start the old GOST version by clicking the DDT version.

      For all others, in particular for the approaching deadline, please read below about the changed version with respect to previous versions.


      The General Observing Setup Tool (GOST)

      Introduction

      The General Observing Setup Tool (GOST for short) is used at the proposal planning stage to specify the anticipated correlator configuration and resources to be used in conjunction with the proposed Karl G. Jansky Very Large Array (VLA) observations.

      GOST is a Java application and is updated for the expected capabilities per VLA observing semester as advertised in the Call for Proposals (CfP, see the current OSS). The use of GOST is required for non-default continuum and spectral line observations in order to ensure that planned correlator setups and data rates comply with the capabilities offered for each VLA observing semester. Proposers who define spectral line correlator configurations must ensure that such configurations are, in fact, allowed and that these configurations are correctly communicated to those reading the proposals. GOST helps determining whether a setup is (in principle) possible and whether it falls under Standard, Shared risk, or Resident shared risk observing.

      The tool enforces most of the known hardware limitations, such as on the number of subbands and number of channels, but does not include the detailed frequency setup for all subbands. GOST does not replace the careful validation that, including the detailed frequency configuration setup checking, is done in the Resource Catalog Tool (RCT). Note that if a correlator configuration can be configured in GOST, it does not mean that it is wise, or that considerations and restrictions outside the correlator are not equally or even more important. If in doubt, create the full setup in a test resource and validate it in the RCT and/or consult the NRAO Helpdesk.

      Proposers must attach the output of this program as a screen snapshot for each of the possible line setups used in the proposal to communicate their intent to the reviewers and the Time Allocation Committee. Continuum and Resident shared risk (RSRO) configurations do not require GOST. Default Continuum resources are integrated into the Proposal Submission Tool (PST) and therefore can be selected directly. RSRO configurations must be described in detail in the appropriate sections of the proposal's Technical Justification.

      In the near future it is anticipated that GOST will be replaced by actual resources as defined in the RCT. The RCT is part of the Observation Preparation Tool (OPT).

      The rest of this document describes running GOST. For capabilities, definitions and other proposing and observing related content please see the current VLA Observational Status Summary (OSS), the Guide to Proposing for the VLA, the Guide to Observing with the VLA, the Proposal Submission Tool (PST) manual, and the NRAO and VLA science web pages in general. For missing or confusing documentation, etc., please contact the NRAO Helpdesk with your queries and/or suggestions.

      Starting GOST

      GOST should only be used to define configurations for Standard and Shared risk observing in non-default continuum and spectral line mode. Plain Continuum can be selected as a default mode in the PST; Resident shared risk configurations must be described in text in the proposal.

      To run GOST, a current version of Java webStart needs to be available on the host computer. The Java issues page has solutions for potential problems. The interactive GOST Java application can be launched (left click) or, alternatively, downloaded (right click) as a jnlp-file and selecting Save link as... In the latter case, GOST can be run without an Internet connection from the command line with javaws <downloadDirectory>/latestGOST.jnlp; this has the disadvantage that an older version of GOST may run if it has not been replaced by the newer version. Usually this is not a problem if the jnlp-file is downloaded just before running it.

      This is the default view of GOST when first launched (click the images to enlarge in a new tab or window).




      Figure 4.1.1: At the top left hand side there is an option menu (Subbands, View and Help) (see figure left), but more importantly there is a configuration designation in colored font in the top center field (see figure above). At the bottom a Save button will save a screen shot to disk which then should be attached to the proposal in the PST (see figure below). Fields shown in blue font are editable and thus under control of the proposer; fonts in black are derived from the selections made.

       

      Configuration Designation

      In the center of the menu bar is a box that contains one of four (five) possible designations for the correlator configuration. The designations are:

      Standard The configuration conforms to the general capabilities of the VLA for this proposal cycle.
      Standard (with Justification) As above, but additional justification needs to be added to the text (currently this only applies to data rates exceeding a certain boundary rate).
      Shared Risk The configuration goes slightly beyond the general and tested capabilities of this proposal cycle and therefore has an element of risk associated with it. Shared risk proposals will not be given the highest scheduling priority due to the unverified nature of the setup, but will otherwise be handled like a general proposal such as being scheduled dynamically.
      Resident Shared Risk This configuration goes significantly beyond the general capabilities of this proposal cycle. NRAO asks that observers proposing with this type of configuration spend a significant amount of time in Socorro to help develop and test this advanced capability.
      Invalid This configuration has a technical problem and cannot be proposed for. When this is the case, the Save button is disabled. This kind of configuration should not be saved nor attached to the proposal in the PST. Please try again.

      For designations other than Standard, the labels of the triggered warnings are displayed in colored bold type with a prefix marker (e.g., "[!]", not shown in subband columns). When this is the case, please use the Why? button (which will appear next to the configuration designation) for more information as to why the software has chosen this designation. It is then left to the proposer to either change the parameters in GOST back to Standard capabilities or to accept the newer designation.

      Saving Your Work

      Clicking the Save button will save a screen shot of the parameters set in GOST to an image file. Please choose a directory location and a unique name for the image file. The system will automatically append a .png suffix. Save one image file for each configuration to be included in the proposal. The PST will accept these images as attachments to the proposal.

      On smaller screens (e.g. laptops) pressing Save in GOST will only save the visible part of the window. If the window does not fully fit the screen, one can use the options under the View menu to adjust the size of GOST: F4 to show only A/C, F5 to show only B/D, F6 to show both A/C and B/D (default). If only a few subbands are filled, one can use the Show Unused Subbands (F7 key) to reduce the size of the screen. Note, however, that the PST only takes a single screen shot as input per resource. If two or more screen shots have to be taken to capture all the information for a resource, one needs to combine them in some other program to a single png before uploading (e.g. in xfig).

      Note that GOST parameters cannot be saved! Whenever GOST is re-started, all the input parameters are set to their default values. If several closely-related configurations need to be constructed, do so in a single GOST session, saving a snapshot of each configuration when completed. Exiting the tool will require to start over again from scratch.

      Special note for users with multiple monitors. Tests in the past have shown problems with the screen capture mechanism for users who have more than one monitor. After saving a GOST screen shot image to a file, please open that file to ensure it took a correct snapshot. If it did not, try moving GOST to another monitor (tests indicate that the first monitor is more likely to give proper results). If the screen capture does not properly work on any monitor, please use a third party tool to take the snapshot and save it in png format only.

       

      Getting Started

      GOST help pages with screen shots follow first. Jump directly to GOST usage hints below.

      Spectral Line Input Help

      GOST allows VLA observers to concentrate correlator capabilities on several narrow regions of frequency space (i.e., in subbands placed in basebands). The VLA offers two different samplers to the community: An 8-bit sampler with 2 basebands of 1 GHz bandwidth each (A0/C0 and B0/D0 for the two polarization pairs), and a 3-bit sampler with 4 basebands of 2 GHz baseband each (A1/C1, A2/C2, B1/D1, B2/D2). While the 3-bit sampler provides more bandwidth, the 8-bit sampler is better suited for high dynamic range experiments. The 8-bit samplers are, per frequency interval (e.g. for spectral line work), also more sensitive than the 3-bit samplers at any frequency.

      For a more detailed description of basebands, subbands, baseline board pairs, etc., and their restrictions for use, please consult the WIDAR section in the OSS, the Spectral line section in the Guide to VLA observing, and the RCT section in the OPT manual.

      The baseband setup fields of GOST summarizes the receiver to be used and the global setup of each baseband property. In the lower portion, the subbands tables (up to 2 by 2 sub-tables), summarize the setup of the subband properties in each of the active basebands.

      Below follows the description and help for the parameter fields of each of these tables.

      Baseband Setup Fields



      Figure 4.2.1: Baseband setup fields: plain 3-bit (top) and 8-bit (bottom) views.

      Receiver Band
      The observing bands labeled with the receiver names are ordered from lowest to highest frequencies. Choose the one to be used for this setup. Note that at this time 4/P-bands are not included in the drop down as only the default P-band setup is offered as non-shared risk mode and this P-band observing setup does not require GOST.

      A/C and B/D Basebands
      The baseband samplers to be used, either 3-bit or 8-bit. Note that mixed settings no longer result in a Shared risk configuration. 8-bit samplers require one baseband center frequency per polarization pair, 3-bit samplers require two (see Baseband Center Freq. next).
      Baseband Center Freq (GHz)
      Depending on the chosen sampler, these are the one or two input fields for the baseband center frequencies in GHz for AC and BD each. For 3-bit, note that the A1/C1 and A2/C2 baseband centers must be separated by less than 2.5 GHz; the same also holds for the B1/D1 and B2/D2 baseband centers. For restrictions in absolute baseband center frequencies between AC and BD, for both the 8-bit and the 3-bit system, see the OSS. Typically only Ka-band baseband center frequency settings turn out to be problematic, but it is best to check anyway.  If a baseband is not going to be used, i.e., remains without any subbands in the subband table, please set the frequency center similar to the one in the other baseband.

      Dump Time (s)
      Enter a time in seconds to select the correlator back-end integration time, which is the resulting visibility integration time. The advised and/or mandatory default integration times are posted in the OSS. To view the defaults as a function of frequency band and array configuration, clicking on [defaults] will open up a separate browser window pointing to the relevant section in the documentation. Any dump time below 50ms will cause this configuration to become a Shared risk configuration. The dump time entered influences the data rate (see Total Data Rate, next), with shorter dump times leading to higher data rates. High data rates may also cause this configuration to become a (Resident) Shared risk configuration as described in the CfP.
      Note that integration times less than the defaults, as discussed in the OSS, require special justification in the text of the proposal. OTF will generally need shorter integrations (dump times) and is a valid justification as such. For OTF, however, special justification is required when the total data rate exceeds the boundaries for Standard, Standard (with Justification), and Shared risk.

      Note that the minimum dump time stated in the CfP assumes a recirculation factor of one. When recirculation is enabled, the minimum dump time will increase with the recirculation factor, e.g., a minimum of 50ms would become a minimum of 200ms if a factor of four recirculation is applied in any of the subbands of the entire setup.

      Total Data Rate
      This is a display field that totals the data rates from the subbands defined in each of the basebands. There are different data rate boundaries at which a Standard setup becomes Standard (with Justification), Shared risk and Resident shared risk. Consult the CfP (and OSS) for the exact limits for each of these boundary data rate limits for this proposal cycle. To easily view the limits as function of designation, clicking on [limits] will open up a separate browser window pointing to the relevant section in the documentation. As the calculations are made for a full array (27 antennas, 351 baselines), for resources to be used in a subarray configuration, the total data rate displayed here should be scaled with the relative allocation of baselines. Notify the NRAO Helpdesk how to handle the GOST image upload when GOST persists in designating a different Shared risk, etc., mode than intended. Note that enabling recirculation may increase the total data rate above current limits.

      Channels x Polarization Products Used
      A display field that illustrates the consumption of the correlator capacity in terms of correlator products by this configuration. The product of the number of polarization products produced for one subband and the number of channels (also known as spectral points) into which that subband is divided, summed over all subbands, should be less than the maximum value of 16,384 (without recirculation). Another way to look at this is to say that summing over all subbands used, the correlator can produce 16,384 channels with a single polarization product; 8,192 channels at dual polarization; or 4,096 channels at full polarization.

      Baseline Board Pairs Used
      A display field that illustrates the consumption of the correlator capacity in terms of baseline board pairs (BlBPs) by this configuration. The correlator consists of 64 pairs of baseline boards that perform the correlations. The value displayed here is the total number of board pairs used by the active subbands in the baseband configurations.

      Baseband Fields/Subbands Table Headers



      Figure 4.2.2: Baseband fields and subbands table header views: plain 3-bit (top) and 8-bit (bottom).

      (Frequency) Range
      With the chosen baseband centers, these are the lowest and highest frequencies covered by this baseband. Basebands are actually either 1.024 GHz (8-bit) or 2.048 GHz wide (3-bit). This is the entire frequency range covered by the baseband and available for subband placement. The correlator does not write out data corresponding to this entire frequency range unless a set of subbands are chosen which cover that entire range (NRAO supplies these default continuum setups and GOST is not required). Note that the baseband edges are not as sensitive as the central part and subbands placed near the edges may not produce the best spectral line observations. If two lines are separated by 900–1024 MHz, it is probably best to choose one line to be observed properly, using a subband in the central part of the baseband, over putting each line at the opposite ends of a single baseband. Remaining basebands may be available to properly observe the other line.

      Data Rate
      The combined data rate for the subbands defined in this baseband. See the description of Total Data Rate above.

      Subbands Table

      Caution! GOST does not require or allow users to specify the center frequencies of the individual subbands. The proposal itself must discuss which lines are to be targeted. Whether a specific frequency setting of individual subbands and basebands is valid can only ultimately be verified by entering the entire setup in the RCT, including all baseband and subband frequency information as well as the Doppler settings for individual sources.

      In the description below, changing a parameter in GOST is done by either selecting from the drop down menu options or by clicking on the relevant blue value of the subband row in the subband table. Adding a single subband in a baseband is done by clicking on the subband index number in the SB column. Deleting a single subband is done by clicking on the subband index number that needs to be removed. If a column is not wide enough to display the full entry value, simply drag the column separator in the header to widen the column of interest.

      Filling/deleting many subbands:
      If the subband setups repeat themselves, one may use the Fill option under the Subbands menu item to bulk fill subbands in basebands. The template menu item selection allows for defining parameters (explained below) for a template subband that can be used to fill a single, all, or part of the basebands with this template; simply close the template setup before using it. Either 16 or 32 subbands can be filled per baseband in 8-bit sampler mode. Using 32 subbands per baseband is not available in 3-bit sampler mode; only the Fill-16 should be used.

      SB (subband index)
      Click the numbered button in this column to either add or remove a subband. NRAO would like the subbands to be entered consecutively. GOST encourages this by deactivating buttons that should not yet be used to add a subband. It does not, though, entirely prevent one from creating configurations with gaps in the table.

      Velo Cov (velocity coverage)
      Displays the approximate total velocity coverage (from the total bandwidth selected in the next column, which equals the number of channels times the channel velocity width) of this subband, adopting the baseband center frequency for the calculation.

      BW (bandwidth)
      Choose a bandwidth for this subband; the widest is 128 MHz, the narrowest is 31.25 kHz. Each option, from widest to lowest, is half the width—or a factor of two—less than the previous. The subband bandwidth is completely independent of the number of polarization products and spectral channels used for the subband, but there are restrictions in combination with recirculation. The maximum recirculation factor is 128 divided by the subband bandwidth in MHz.

      Prod (polarization products)
      Choose from the list. Full means all four polarization products (RR, RL, LR, LL); dual means the two parallel-hand polarization products (RR & LL). Changing this value will change the number of spectral channels available without changing the number of baseline board pairs or recirculation factor: single polarization gives twice as many channels as dual, and dual gives twice as many as full. Per baseline board pair without recirculation, the number of channels is the product of the number (1, 2 or 4) of polarization products and 64, or the equivalent to 256 divided by the number of polarization products.

      Recirc (recirculation factor)
      Choose from the list. Recirculation is a term to describe the method to increase the number of spectral channels using correlator software as opposed to baseline board stacking which uses correlator hardware. Recirculation is currently achieved by limiting the subband bandwidth, as opposed to visibility integration time, and only available for subbands less than 128 MHz wide; the maximum recirculation factor for a subband is 128/(subband bandwidth in MHz).
      By default, GOST will not enable recirculation: the entry in the column labeled Recirc is 1. Recirculation higher than one, in powers of two, can be used for subband bandwidths less than 128 MHz to increase the number of channels (spectral resolution) without straining the hardware limits imposed by the maximum number of baseline board pairs (see below). Recirculation up to a factor 64 is Standard and subject to other configuration restrictions such as total data rate. For help with recirculation, please contact the NRAO Helpdesk.

      BlBP (baseline board pairs)
      Choose from the list. It displays the number of baseline board pairs (BlBPs) dedicated to this subband (with a minimum of 1). This number of BlBPs is a priori allocated by this subband and cannot be shared with other subbands. See the description of Baseline Board Pairs Used (above) or the general correlator capabilities in the OSS. The number of BlBPs used may affect the number of subbands available in this or any other baseband. This is correctly handled by GOST. See the OSS for details if this is an issue. Note the difference between BlBP stacking and recirculation to increase spectral resolution (number of channels): recirculation is preferred over stacking as, in principle, less hardware is used and thus may provide more individual subbands and/or help in the scheduling logistics.

      Ch Wd (v) (channel width in approximate velocity)
      Displays the approximate width of a single channel of this subband in units of velocity, calculated for the center of the baseband with the selection of bandwidth, recirculation, and baseline board pairs. Actually, this is the separation between adjacent spectral points, before any spectral smoothing—these channels are not truly independent.

      Ch Wd (f) (channel width in frequency)
      Displays the fixed frequency bandwidth of a single channel of this subband with the selection of bandwidth, recirculation, and baseline board pairs. Again, this is the separation between adjacent spectral points, before any spectral smoothing—these channels are not truly independent.

      Channels (spectral points)
      The values represent the number of spectral points per polarization product into which this subband will be divided. Note that the number of polarization products, recirculation factor, and number of baseline board pairs chosen in the columns to the left all influence the final value.

      MB/s (data rate in MB/s)
      Displays the data rate associated with this single subband. See the description of Total Data Rate above.

      Figure 4.2.3: Example of a Standard (with Justification) setup. Note that clicking the Why? button (in GOST) next to the green Standard (with justification) configuration designation will expose why the justification is needed (in this case because data rate exceeds the Standard limit of 25 MB/sec).

      GOST Usage Hints

      Setting up a resource in GOST is simply done by first modifying the template (Menu -> Subbands -> Template) and activating the individual subbands by clicking on the subband ID number in the leftmost column (top to bottom). The activating of individual subbands is done after selecting the receiver band, baseband samplers (3-bit or 8-bit), and baseband center frequencies in the active basebands. The latter can be adjusted at any time, as can the dump time (visibility integration time). Default dump times for the different array configurations can be viewed by clicking the [defaults] link which refers to information maintained in the OSS.

      Figure 4.2.4: Shown left is the default template subband view. Design your template by modifying the columns with the blue headers to obtain the required channel frequency width. It may be useful to first fill in a subband in the subband table to derive the parameters at the observing frequency for that baseband (i.e. converting frequency into velocity). Close the template window when done; each new active subband will be filled with this template.

      Then, per subband created, select (click) the bandwidth, required polarization products, and number of channels in which the subbands needs to be divided. Repeat (or see below) until all active subbands are defined. If the product of the polarization products and number of channels in a single subband is larger than 256, more than one baseline board pair (BlBP) is allocated. If the total number of BlBPs allocated reaches 64, and not all exposed subbands are activated/defined, no additional subbands can be added. If more subbands are required then reduce the number of BlBPs in select subbands to enable recirculation (see above). Recirculation allows use of more channels given limitations in hardware, but this may designate the configuration away from Standard for higher factors of recirculation.

      If many subbands with almost the same setup are required, the use of a template subband is recommended. The template is used as default for every new subband that is activated. When whole basebands need to be filled out with subbands similar to the template, the suggested setup sequence becomes:

      1. Set up a template subband:
      • In the upper left corner click: Subbands -> Template
        • select a bandwidth
        • select the polarization products
        • select the number of channels
      • Perhaps adjust the number of BlBP (which enables recirculation!)
        • close the template window
    • In the upper left corner click: Subbands -> Fill -> [subbands to fill, e.g., A1/C1]
      • this fills as many subbands with the template as allowed (16 subbands/baseband, 64 BlBPs)
    • Adjust individual subbands for select properties.
    • Check the configuration designation. If not the desired configuration designation, adjust parameters or start anew until happy.
    • If subbands and BlBPs remain (and within the desired total data rate), create additional (template) subbands, check configuration designation, adjust parameters or start anew until done (and happy).
    • When done with a GOST setup, do not forget to click the Save button! Also, when a new setup to be created resembles this previous one, do not exit GOST, but continue after save to secure the image for the PST attachment, and adjust the current view.

      If GOST Fails...

      It is not impossible that GOST fails to configure or mis-designate a specific complicated spectral line setup, or even has problems with a mode that is advertised as Standard or Shared risk. In such a case, the ultimate reference of the possible capabilities is the RCT. The RCT knows the details about frequency tuning (not known to GOST, so especially for Ka band, it is worth checking in the RCT) as well as some more complicated correlator resource allocation details that are not available to GOST. When a setup is required that is not handled properly by GOST, please let the NRAO Helpdesk know. In such a case, it is acceptable that a png snapshot of the Validation Tab is uploaded to the PST instead of a GOST image. Note that, as with small monitor screen shots above, only one png can be uploaded per resource; some image gluing may be needed to provide the whole Validation Tab page information.

      7. Mosaicking

      Mosaicking

      The VLA supports mosaics that use a discrete pointing pattern with pointing centers that are set up as individual fields to be observed (as if they were just a set of target sources).

      For more information on setting up mosaicking, please see the section on Mosaicking and OTF in the Guide to Observing.

       

      On-The-Fly Mosaicking (OTFM)

      OTF mosaicking eliminates the slew and setup overheads and thus is useful for shallow very large mosaics. OTF mosaicking with the VLA is currently offered under the SRO program only.

      For more information on setting up on-the-fly mosaicking and various practical considerations to request observing time in this mode, please see the On-The-Fly (OTF) Mosaics subsection in the Mosaicking and OTF section of the Guide to Observing.

      8. Exposure and Overhead

      Time on Source: Exposure Calculator

      The VLA observer is responsible for observing all calibrators required to properly observe as well as calibrate the data after observation in the allocated telescope time. The total time a proposer requests has to include not only the time on the sources of interest, but also time spent observing calibrators, slew time between sources, and various types of setup times. The VLA Exposure Calculator is a web-based tool (https://obs.vla.nrao.edu/ect) to help observers to perform these approximate calculations.

      If you have an old JAVA version of the exposure calculator, please discard it and do not use it. Please make sure you use the latest web-based version of the calculator.

      For the 18A version of the exposure calculator, there may still  be some messages that appear that are bogus, in particular ones about "Below brightness temperature limit". This message also mentions something about the confusion limit. For A  configuration, even at low frequencies, confusion should not be a problem.  For D configuration, of  course, confusion could be a problem at low frequencies.   There are also times when a message appears about a setup spanning a frequency range larger than the receiver bandwidth, even when it's clear that the setup does not do that. If there are concerns about any messages from the ECT,  or if you have a question about the confusion calculation in D configuration setups, please send a message to the NRAO Helpdesk.

      The Exposure Calculator performs the following three types of calculation:

      1. given bandwidth, sky frequency, image weighting, number of polarizations, and RMS noise required the ECT will return time on source
      2. given time on source, bandwidth, image weighting, number of polarizations, and sky frequency the ECT will return RMS noise
      3. given time on source, RMS noise required, image weighting, number of polarizations, and sky frequency the ECT will return bandwidth

      The calculator essentially solves the image noise equation given in the Sensitivity section of the VLA Observational Status Summary (OSS).

      Running the Calculator

      Important:

      • The fields labeled Representative Frequency and then Bandwidth must be entered before the calculator will do anything else.
      • After entering data into a field a carriage return <cr>, a <tab>, or a mouse click outside the input field will submit the data to the calculator.
      • By hovering the mouse over some fields, a tool tip with some helpful information is shown.

      The following screenshot shows the various fields, a description of which is included further below.

       

       

      Description of the various fields:

      • Array Configuration (A, B, C, or D): does not affect the RMS noise, but does indicate the brightness temperature sensitivity as well as the confusion level that will be reached (see the OSS for further discussion of confusion).
      • Number of Antennas: many use 25 instead of the full total 27 antennas to allow for the contingency that not all telescopes are in working order. For RMS calculations per baseline (e.g., for calibration) enter 2 for the number of antennas. See the Calibration section in the Guide to Observing with the VLA for more information.
      • Polarization Setup: either single or dual polarization products. Typical value would be dual for Stokes I flux (density) measurements.
      • Type of Image Weighting: this is the weighting of the data in the u-v plane during imaging. This affects the RMS noise sensitivity and the beam. Natural weighting is usually used to obtain the best sensitivity and a larger, somewhat worse beam, in terms of angular resolution and sidelobes. Robust (= 0) imaging gives somewhat less sensitivity but a somewhat smaller and better beam as compared to Natural. Use Natural for detection experiments.
      • Representative Frequency: the observing frequency (not the rest frequency) that determines the observing band. The online dopset tool can be used to convert rest frequencies to on-the-sky frequencies at the VLA for certain observing dates.
      • Approximate Beam size: This is the approximate synthesized beam size which is based on the frequency, the configuration selected, and the type of weighting. If the bandwidth divided by the frequency is greater than or equal to 0.25, a range of beam sizes is shown that correspond to the range in frequencies.
      • Digital Samplers: 3-bit or 8-bit can be selected. If the bandwidth is wider than 2048 MHz, the calculator will automatically set the samplers to 3-bit, with 8-bit not selectable. Bandwidths up to 2048 MHz cause the calculator to default to 8-bit samplers, although 3-bit sampling may be selected if desired. Note that, in general, 3-bit is less sensitive to lines for a given time and requires more overhead time. For more information, see the VLA Samplers section in the OSS.
      • Elevation: the RMS noise, especially at high frequency, is worse at lower elevation mostly due to the atmospheric opacity increasing the system noise. We calculate the increase in system noise assuming an average opacity selected by season (see Average Weather below). The elevation goes into the calculation as the standard exponential of the opacity multiplied by the secant of the elevation. The 4 elevation selections for calculation purposes are:
          • zenith, elevation = 90 degrees;
          • high, elevation = 60 degrees;
          • medium, elevation = 40 degrees;
          • low, elevation = 18 degrees.
        • Note that for S-band (2–4 GHz), ground spillover results in higher system temperatures; S-band observations below 20 degrees elevation are not recommended.
      • Average Weather: this entry selects the empirical opacity based on many years of tipping data at the VLA, as discussed in VLA memo 232 and EVLA memo 143. What is actually selected is the 22 GHz opacity; opacities at other frequencies are derived from this. The average weather corresponds to 22 GHz opacities as follows (no diurnal variations in the opacity are accounted for): 
          • Summer: 22 GHz opacity = 0.158
          • Autumn: 22 GHz opacity = 0.07
          • Winter: 22 GHz opacity = 0.045
          • Spring: 22 GHz opacity = 0.091
      • Calculation Type: see the three main operation modes described in the ECT introductory section.
      • Time on Source: input or output field. This is the time on the source of interest, not including overhead for calibration, slewing, etc. Also see the description of the next field, Total Time.
      • Total Time: input or output field. This is the total time to be requested in the proposal, and includes typical additional calibration, slewing, etc., for a single target. Default overhead values, as a function of receiver band and array configuration, have been implemented into the time or noise calculations and are based on a 2-hour scheduling block. As the actual length of the block can vary widely, these values should be viewed as guidelines only. It does not specifically account for the fixed 10-13 minute startup overhead that should be added to every session. Also, as noted in the Overhead section below, the overhead is different (longer) for 3-bit observations as compared to 8-bit observations. The overhead values in the calculator are closer to the 3-bit values. Both Time on Source and Total Time, which includes the single source logistical overhead, are displayed (and can be input) in the calculator. Also, in the Overhead section, we provide guidelines on how overhead time is calculated assuming a 2-hour scheduling block. Time on Source or Total Time takes various input formats, e.g., 20s (for twenty seconds), 10m 10s (for ten minutes, ten seconds) or 2.5h (for 2.5 hours). Time inputs can also be just seconds, minutes, or hours (e.g., 800s, 75m), or 2h 5m 35s (for 2 hours, 5 minutes, 35 seconds). Spaces between units are optional.
      • Bandwidth (Frequency): the bandwidth in frequency units. For continuum use the full usable bandwidth (excluding RFI), up to 2 or 8 GHz. For spectral line observations, one can use the width of a channel or the width of many channels that define the RMS in the science goal. This can be the total anticipated width of a line or a fraction thereof. In any case, please mention and explain the bandwidth value that is used in the Technical Justification

        At lower frequencies, RFI can limit the amount of effective bandwidth for observing. The calculator does not take this into account in its calculations, so it is up the observer to insert a reasonable bandwidth for continuum observation calculations. The maximum affected bandwidth for the lower frequency bands (L through Ku-band) is given below, and also as messages in the calculator:

        • L-band (1–2 GHz):  maximum affected bandwidth 40% (i.e., 600 MHz is a reasonable effective total continuum bandwidth)
        • S-band (2–4 GHz):  maximum affected bandwidth 25% (i.e., 1500 MHz is a reasonable effective total continuum bandwidth)
        • C-band (4–8 GHz): maximum affected bandwidth 15% (i.e., up to 3.4 GHz is a reasonable effective total continuum bandwidth when using 3-bit samplers)
        • X-band (8–12 GHz): maximum affected bandwidth 15% (i.e., up to 3.4 GHz is a reasonable effective total continuum bandwidth when using 3-bit samplers)
        • Ku-band (12–18 GHz) maximum affected bandwidth 12% (i.e., up to 5.28 GHz is a reasonable effective total continuum bandwidth when using 3-bit samplers).
      • Bandwidth (Velocity): the bandwidth in velocity units (assuming the line is at redshift zero). For continuum, use the full usable bandwidth (excluding RFI) up to 2 or 8 GHz in the field above (Bandwidth Frequency) instead of this field. For line observations, one can use either the field above in frequency units or this field in velocity units (at z=0). See Bandwidth Frequency above for what to enter.
      • RMS Brightness Temperature: the conversion to RMS brightness temperature from RMS flux density depends on the size and shape of the synthesized beam. Since details of the actual (u,v)-coverage are unknown, we have to make certain reasonable assumptions about the beam shape; here we assume that the beam is Gaussian and round. This means the derived RMS brightness temperature is approximate only. More details about this conversion can be found on our mJy/beam - Kelvin conversion page.
      • Confusion Level:  Given the array configuration (i.e. synthesized beam), and the sky frequency, the confusion level is calculated.  This confusion level is displayed here.
      • HI Column Density: this feature is only for science projects targeting neutral Hydrogen and is shown in the calculator when the frequency is at L-band and < 1500 MHz. The calculation assumes a rectangular line shape of width given in the bandwidth entry and calculates an HI column density based on the RMS value.
      • Help: clicking this button brings you back to this page.
      • Save: a screen capture to a PDF file is available (the Save button at the very bottom of the calculator). This PDF can be uploaded to the proposal's Technical Justification in the Proposal Submission Tool (PST). Before uploading, please check the PDF file for any errors (which may be caused by timeout of the web based exposure calculator tool) and compare the numbers in the PDF with the text input fields in the proposal Technical Justification and possibly the Scientific Justification.

       

      A Special Case: P-Band

      The exposure calculator has been updated for P band (236–492 MHz) observations. Elevation and seasonal differences have been turned off for this frequency. The noise calculation for P-band, however, is done assuming reasonably high Galactic latitude (greater than 60 degrees). At low Galactic latitudes, the Galactic sky background dominates the system temperature and the exposure calculator does not take this into account. We have made some initial tests on how much time over what the exposure calculator reports is needed for low Galactic latitudes. These are rough estimates: for Galactic latitudes below 30 degrees one should increase the time request over the exposure calculator by a factor ~2; for latitudes between 30 and 60 degrees by a factor ~1.2. For observations at the Galactic Center (a special place), the time might need to be increased by a large factor ~30. We are continuing to try to improve the time estimates and noise calculations for P-band. Questions about P-band observing should be directed to the NRAO Helpdesk.

       

      Overhead and Total Time

      Every proposal needs to specify the total amount of time requested which includes setup scans, slewing, and observations of calibrators. Using the information supplied in the input fields, the exposure calculator first derives a Time on Source. It then uses the array configuration and the observing band to make a best estimate of the required overhead to arrive at a Total Time for a phase-referencing type of observation to reach the sensitivity requested. The sum of the Total Time with the fixed scheduling block (session) overhead, and any additional overheads described below, is the observing time request that should be specified in the proposal. N.B. The calculations of total overhead depend upon the length of the scheduling block; such calculations done later in this section and in the exposure calculator assume a two-hour block.

      Types of Overhead

      The main types of overhead are:

      Fixed.  The start-up scan sequence needs to be at least 10 minutes (lower frequencies) or 13 minutes (when reference pointing is used) for setup scans and for slew time from the previous (unknown) pointing. Another type of fixed overhead is that there usually needs to be one flux/bandpass calibrator scan per observation. This fixed overhead obviously affects the shorter scheduling blocks (0.5 and 1 hour) more than the longer ones.

      Fractional.  This is largely determined by periodic complex gain calibrator scans, slew time in between target source and complex gain calibrator, and slew time between target sources (if more than one). High frequency observations require more frequent source/calibrator sequences and therefore require more fractional overhead. They also require reference pointing scans, typically once every hour.

      Thus, for any frequency observing, the overhead involves:

      • A 10- (no reference pointing) or 13- (reference pointing) minute block of setup scans at the start of the observation to ensure to get on source;
      • A flux/bandpass calibrator scan of duration 5–10 minutes, depending on its brightness and position with respect to the target fields;
      • Complex gain calibrator scans, each long enough to detect it (typically ~1 minute duration), and often enough for phase coherence, e.g., once every 30–60 minutes for the low frequencies (see the Low Frequency Strategy in the Guide to Observing with the VLA) and as fast as every 2 minutes at the higher frequencies (see below);
      • Slew time between source changes, that is, twice during a cycle time;
      • a 30 second requantizer scan whenever a scan uses a 3-bit resource different from the resource used in the previous scan (whether 3-bit or 8-bit).

      For high frequency observing, in addition to all points above, we also have:

      • A 3–4 minute reference pointing scan once every hour or with each large (>20deg) angular displacement in the sky;
      • More frequent complex gain calibrator scans, from once every minute (fast switching) to once every ~10–15 minutes when relying on self-calibration (see the High Frequency Strategy in the Guide to Observing with the VLA);
      • Increased slewing overhead because of more frequent source changes.

      Then there are special cases which require even more overhead, e.g., polarization calibration, or multiple bandpass calibrator scans.

      Estimating Overhead

      Based on the array configuration and the observing band, the Exposure calculator multiplies the Time on Source with a multiplication factor (factor in the table below) to arrive at a total time, according to the following table:

      Table 8.1: Source Scan Length Total Time
      A,B configurationC,D configuration
      band(s) factor source scan length
      factor source scan length
      4,P 1.154 50 1.154 50
      L,S,C,X 1.261 30 1.261 30
      Ku 1.885 10 1.885 10
      K,Ka,Q 2.747 2 1.885 10

      These numbers were determined by creating realistic scheduling blocks with the scan length on source as shown in the table. A number of assumptions were made:

      • Scheduling block duration: 2 hours. Overhead will decrease slightly for longer scheduling blocks, but can increase substantially for shorter blocks.
      • Source scan duration as shown in the table, calibrator scan duration 1 minute.
      • 3-bit observing for the higher frequency bands, and 8-bit observing for the lower frequency bands.
      • Referenced pointing for Ku-band. If observed without using referenced pointing, 1.5 may be a better value.

      Note: every case is different, and the entries in the table (and therefore the Total Time reported by the Exposure Calculator) are only guidelines. Our recommendation, especially for higher frequencies, is to determine your overhead empirically by creating a realistic test scheduling block in the OPT with the on-source time to achieve the science goal. Experiment with different LST start times and use the most reasonable total time reported by the OPT.

      Data Volume

      Using the total estimated observing time (on source + overhead) and the data rate which is given in the default resources in the PST or in GOST, the total data volume of the proposed observations can be computed. The VLA OSS provides more details on the data rates and limits.

      9. Technical Justification Example

      An important part of the proposal is the Technical Justification, which is a separate element of each proposal. The proposers are asked to supply information on a number of standard, potentially important, issues covering a wide range of technical and logistical aspects. This reduces the likelihood that information on specific considerations to judge the feasibility of the proposed observations is lacking. The Technical Justification section presents guiding questions and accompanying web links point to more information on the individual topics (in the actual PST section, not in the image below). Note that for every (relevant use of each) setup a PDF file of the Exposure Calculator needs to be uploaded.

      Since any actual Technical Justification depends strongly on the combination of science goal, the instrument configuration, etc., it is difficult to give general guidelines. The example below is for a single case of extragalactic HI observations.

      10. Submission Guidelines

      The following guidelines are designed to assist in the submission of proposals into the Proposal Submission Tool (PST). There are two sets of guidelines, one for General and Shared Risk Observing (GO / SRO) and one for Resident Shared Risk Observing (RSRO); note that while some of the steps to completion are the same between GO / SRO and RSRO, the latter has additional considerations that need to be taken in account before submission of the proposal.

       

      General and Shared Risk Observing (GO / SRO) Steps to Completion

      1. Read the current Call for Proposals description for summaries of the capabilities being offered for GO and SRO.
      2. Write a scientific justification for your proposal. Note that technical information should be included in the Technical Justification section form and does not need to be included with the scientific justification. Save your scientific justification as a PDF file.
      3. If you are proposing for spectral line observations, run the GO Setup Tool (GOST, a Java application) to define your correlator setup and save a snapshot of the tool GUI to disk. If you have trouble running GOST please contact the NRAO Helpdesk.
      4. Use the exposure calculator to determine the required sensitivities and requested total time, including all overheads.
      5. Log into the NRAO Interactive Services page and click on the Proposals tab in the top left to create a new proposal for the VLA. Once you are in the tool, extensive help is available in the tool by clicking on the Help button in the top right of the tool interface. In the Proposal Submission Tool (PST):
      • Fill in the relevant fields for each section of your proposal;
        • If you are proposing for continuum observations, select Observing Type = Continuum in the General section of your proposal. Then add a Continuum Resource, selecting the appropriate band you want to observe in. Default optimum continuum setups for each band are defined in the PST.
        • If you are proposing for spectral line observations, select Observing Type = Spectroscopy in the General section of your proposal. Then add a Spectroscopic Resource and attach the GOST snapshot (that you previously saved) to this resource. The GOST snapshot describes everything needed to specify the lines you want to observe and what correlator resources are needed for the observation.
      • Upload your scientific justification as a PDF file;
      • Technical justifications are now included as a separate section in the proposal. Click on the Technical Justification page in the proposal and fill in the appropriate boxes (example of technical justification). In order to determine sensitivities, you will need to use the VLA Exposure Calculator Tool (ECT) to understand what sensitivity you will get for a given frequency, bandwidth, and integration time (guidelines for running the ECT). If you have trouble running the ECT please contact the NRAO Helpdesk;
      • Note that the PST allows observers to specify multiple resource types (e.g., you can have one proposal that specifies general and/or RSRO correlator resources). If any resource is RSRO then the proposal will become a RSRO proposal.
    • When your proposal is complete, validate it to make sure there are no obvious omissions or mistakes. Also double check the numbers used in the Scientific and Technical justifications with the ones in GOST and ECT attachments.
    • When you are satisfied, submit your proposal in the PST. Note that proposals can be withdrawn and resubmitted before the deadline.
    •  

      Resident Shared Risk Observing (RSRO) Steps to Completion

      1. Read the current Call for Proposals for a summary of the capabilities being offered for GO and SRO. If you want more than what is offered for GO or SRO then you are requesting a RSRO capability (one that is not well-tested or may even need additional development). If you propose for a RSRO capability, you—or an experienced person on your team—must be able to participate in the RSRO program by coming to Socorro to help with the development and testing process.
      2. Write a scientific justification for your proposal. Note that technical information should be included in the Technical Justification section and does not need to be included with the scientific justification. Save your scientific justification as a PDF file.
      3. Log into the NRAO Interactive Services page and click on the Proposals tab in the top left to create a new proposal for the VLA. Once you are in the tool, extensive help is available in the tool by clicking on the Help button in the top right of the tool interface. In the Proposal Submission Tool (PST):
        • Fill in the relevant fields for each section of your proposal;
        • Even as a RSRO proposal, you will need to create a Resource in the PST and select the WIDAR RSRO Back End. This will give you a text field in which you can type a description of your setup;
        • Upload your scientific justification as a PDF file;
        • Technical justifications are now included as a separate section in the proposal. Click on the Technical Justification page in the proposal and fill in the appropriate boxes (example of technical justification). Describe what you want to do with the correlator and why this is in the RSRO Category in this section. Contact NRAO staff if you have questions about exactly what is feasible. The last box at the bottom of the Technical Justification page should be used to describe your RSRO effort. Identify who in your team can come to Socorro to help commission this capability and how their background and expertise can be applied to this development. Working with NRAO staff, estimate the level of effort that is likely to be needed for this development and specify how long a member of your team can come to NRAO.
        • Note that the PST allows observers to specify multiple resource types (e.g., you can have one proposal that specifies general, shared-risk and RSRO correlator resources). If any resource is RSRO then the proposal will become a RSRO proposal.
      4. When your proposal is complete, validate it to make sure there are no obvious omissions or mistakes.
      5. When you are satisfied, submit your proposal in the PST. Note that proposals can be withdrawn and resubmitted before the deadline.

       

      RSRO Considerations

      A RSRO proposal should contain:

      1. A scientific justification, to be peer reviewed as part of NRAO's current time allocation process, submitted through the Proposal Submission Tool. Note that RSRO correlator resources should be specified as plain text on the Resources page in the PST by selecting WIDAR RSRO as the backend.
      2. The technical justification should identify the personnel who will be involved in the residency and describe how their expertise will be used to address the critical priorities of VLA development relating to their proposal. The proposed dates of the residency must be included, so that the residency can be matched to VLA development planning. This section will be reviewed by NRAO staff. Limited support for accommodation in the NRAO Guest House for participants in the RSRO program may be available.

      The acceptance of a RSRO proposal will depend on the outcome of the time allocation process. Proposals will also be evaluated by NRAO staff in terms of the priorities and benefits to the VLA development and commissioning activities.

      In general, one month of resident commissioning effort is expected for every 20 hours of VLA time awarded to a RSRO project, subject to negotiation. There is no minimum requirement for the amount of residency at NRAO. However, the amount of time spent at NRAO to help develop the program should be realistically matched to the expected effort, including time to become familiar with relevant technical aspects. The time proposed at NRAO should be discussed with NRAO staff to determine what is reasonable. The length of time a RSRO expert should be needed at NRAO may be on the order of a few months.

      The period(s) of residency may occur in advance of the observing time awarded in order to decouple essential scientific requirements (such as array configuration) from other factors which may affect when personnel are available (such as teaching schedules). However, observers should be present for one week prior to their observations in order to become familiar with the latest developments and to set up their observations. In the special case of Target of Opportunity proposals, a VLA staff collaborator may be required for setting up observations on short timescales.

      It is possible for a member of the NRAO scientific staff to satisfy the residency requirement on a RSRO proposal. NRAO staff considering providing the residency requirement for an RSRO proposal should consult with their supervisor for further information. Graduate students may satisfy the residency requirement, provided relevant expertise is demonstrated in the RSRO relevant sections of the proposal. Graduate students should be accompanied by their advisor at the start of their residency. Resident personnel will work under NRAO management in order to optimize the overall effort. A set of clear goals will be agreed upon in advance of the start of the residency.

      The types of proposals considered under the RSRO program may include both large (>200 hours) and small (~10–200 hours) projects. Qualified large projects proposed by consortia will be considered as long as the residency requirements are met. A single individual may satisfy the residency requirement for several small projects.

       

      RSRO participation without a science proposal

      In some cases an individual may want to participate in development activities without writing a science proposal. A participant may arrange to visit Socorro to contribute to development activities by submitting a proposal of who will come and the technical development to be undertaken directly to the Assistant Director for NM Operations (nraonmad@nrao.edu). If the Assistant Director approves of the request, then the individual may come to Socorro to contribute to development activities. The participant may then obtain observing time either by submitting a proposal at a regular proposal deadline, or by submitting an Exploratory Proposal through Director's Discretionary Time. Such visits should conform to the residency requirements above. Proposals to visit Socorro under this program may be submitted at any time.

      11. Frequently Asked Questions

      Noise

      Bandwidth

      What bandwidth should I use for noise calculations?

      If you are interested in the noise per channel, as you would be in most spectral-line projects, use the bandwidth of one channel to give you the noise per channel. If you are are doing a continuum detection experiment, in which you want to cover as much spectral range as possible, use the total (RFI-free) bandwidth. When in doubt, use the total bandwidth you plan to use in making an image.

      Polarization Products

      How does the number of polarization products affect the noise?

      Choices are single polarization (RR or LL), dual polarization (RR and LL), and full polarization (RR, RL, LR, and LL). If the emission is unpolarized, dual polarization gives twice the amount of independent data as single polarization does, and therefore decreases the rms noise by a factor √2, as the sensitivity calculator will show. Full polarization does not improve this any further since RL and LR do not contribute to Stokes I.

      Basebands

      Can I decrease the noise by overlapping the basebands?

      No! For instance, in the case of the 8-bit samplers there are two independent baseband pairs. These are simply two windows in frequency space on the same data stream. Having the two windows look at the same data does not affect the measured rms noise.

       

      When to Use Overlapping Basebands

      So there is never any reason to use overlapping basebands?

      Actually, there is. Often basebands are divided up in a number of subbands (e.g., 8), each of which has reduced sensitivity at the edges. So in the above example there are a number of frequency ranges with reduced sensitivity. By overlapping the basebands by one-half of one subband width, ranges with reduced sensitivity in one baseband will have uncompromised data in the other.  For more details, see the Spectral Line section within the Guide to Observing with the VLA.

       

      Overhead

      What overhead should I count on for slewing, calibration, etc?

       

      How to tune Basebands

      How should I tune the two baseband pairs of the 8-bit samplers? Similar considerations may apply to the 3-bit samplers that provide four baseband pairs.

      For the 8-bit samplers, the WIDAR correlator processes two independently tunable baseband pairs. There are a number of common applications; which one to choose is determined by the science you want to achieve.

      • Tune end-to-end. This is often done if one wants to obtain as wide a contiguous bandwidth as possible. For instance, one 1 GHz baseband pair is tuned to 4.5 GHz, and the second to 5.5 GHz, for a complete 4–6 GHz coverage.
      • Tune as far apart as possible within one band. Often used for spectral index determinations; for instance one 1 GHz baseband pair is tuned to 4.5 GHz and the second to 7.5 GHz, to obtain 4–5 GHz and 7–8 GHz coverage.
      • Tune to target different spectral lines; for instance, one 8 MHz subband from a given baseband pair is tuned to the NH3(1,1) line at 23.695 GHz and a second 8 MHz subband from the other baseband pair to the NH3(5,5) line at 24.533 GHz. The basebands are not contiguous, but each covers the line of interest with the desired frequency resolution.
      • Tune the two baseband pairs such that they are shifted by a fraction of one subband (i.e., largely overlapping). Each baseband pair consists of a number of subbands, and there is sharply decreased sensitivity at subband boundaries. When tuning the second baseband pair one-half of a subband width away from the first baseband pair, compromised data at a subband boundary in one baseband pair can be replaced by good data in the second baseband pair.

      Note that the two baseband pairs offer two 'windows' on the available spectrum. If the baseband pairs overlap, the data in the overlapping part of the spectrum will be essentially identical in either baseband pair. This is why tuning both baseband pairs to the same frequency does not increase the S/N by √2.

       

      Setup Restrictions

      How do current observing setup restrictions impact my proposal?

      With the WIDAR correlator, there are a number of observing constraints. Most of these become important when preparing the observing script, but the following are of potential interest to proposers as well.

      • Ka-band (26.5-40 GHz) has the most restrictive tuning restrictions. Only half of the available basebands can be tuned below 32 GHz, and tuning around 32 GHz might be problematic. Further information on Ka-band tuning can be found in the current Observational Status Summary section on VLA Frequency Bands and Tunability.
      • Scheduling blocks (SBs) must use the appropriate setup scans for 8-bit and 3-bit resources. This will add to the overhead.
      • If a target is observed at multiple frequencies there is increased calibration overhead since each target source scan at a certain frequency has to be bracketed by gain calibrator scans at that frequency.
      • The SB Validation checklist (located in the Guide to Observing with the VLA) is important to follow before and after scheduling block creation.

       

      GOST does not start

      GOST does not start up when I click its link.

      This can happen if you don't have the correct webstart installed. In that case we recommend you start GOST using our alternative route.

       

      Fast Switching

      I used fast switching with the old VLA; how do I do it now?

      Fast switching in the old VLA was a way to reduce the slewing and setup overhead compared to traditional iterating between source and calibrator scans. In the new online system this overhead is sufficiently reduced that no special fast switching mode is necessary; its role is now taken over by a regular source - calibrator loop.