Interferometric pointing (a.k.a., reference pointing) may be needed at frequencies of about 15 GHz and higher (K, Ka, Q-band, and most Ku-band setups). At these frequencies the antenna pointing accuracy (a few arcseconds) becomes a significant fraction of the primary beam. Observing with an inaccurate pointing thus may degrade the signal by a significant fraction. The antenna pointing is a function of the shape of the reflective surfaces and is influenced by, amongst other things, gravity and temperature. Therefore, observing at high frequencies may require regular pointing scans to determine offsets from the pointing model. These pointing offsets remain reliable for target sources within about 20d in AZ or EL from the pointing position. Therefore, typically an observation would redetermine pointing solutions when moving to a different portion of the sky, or roughly hourly when tracking a (group of nearby) target source(s).

Pointing scans are performed as a five-point raster observation on a strong (over 300 mJy) continuum calibrator, in first instance in X-band continuum. This primary reference pointing scan usually yields sufficiently accurate pointing offsets, but if more accurate solutions are required a secondary reference pointing may follow at the (standard) frequency of the observing band. Secondary pointing is also performed in continuum mode (to be as sensitive as possible to the continuum source) in an attempt to improve the antenna pointing in the band of interest. However, local lore is that although this might improve the pointing a bit toward the pointing source, subsequent slews and with time passing by, this secondary pointing in general does not yield a long lasting improvement on the primary pointing. In addition, there is the risk that for some antennas the secondary solution fails. The resources for secondary pointing scans are available, but it is debatable whether the extra time spent to perform a secondary pointing scan is worthwhile. Determining pointing solutions using spectral line sources, e.g., with SiO masers in Q-band, has not been tested.

Default pointing resources are included in the NRAO defaults catalog in the group Pointing setups. You may want to copy the X-point (formerly known as the X band pointing) resource and pointing sources you wish to use from the standard catalogs to your personal catalog.

Setting Up an IP Scan

To utilize the interferometric pointing scan mode, select Interferometric Pointing from the drop-down menu under the scan mode section of a scan (Figure 4.21). These scans should use a long enough scan duration (in LST) to include an on source time of at least 2min 30sec. It is advised to start a block of high frequency observations with a pointing scan, and tick the apply reference pointing for each scan thereafter. This tick-box will apply the offsets that were determined in a previous pointing scan; if you forget you will be using the (most likely less accurate) default pointing model. Your first calibrator scan (after the setup scans) may be a pointing scan, but as you don't know in what AZ the array starts, you should allow for ample slewing time or anticipate a worst case scenario using the AZ starting conditions on the Reports tab.

Figure 4.21: Example of an Interferometric Pointing scan used in an SB.

If the pointing scan has not finished by the stop-time of this scan, no valid solutions can be applied. If it has determined a pointing solution before the stop-time has been reached it will continue with another five-point raster, which may or may not yield new solutions (which will be averaged with the first raster solutions). For secondary reference pointing scans, apply the solutions of the preceding primary reference pointing scan.

A pointing scan is for real-time calibration and, while very useful for real-time calibration, usually does not yield useful data for your project. The data is included in the observations, however, you will need special input parameters to load the data in your data reduction package (CASA or AIPS). You may study this data for reference, but the real-time corrections are already applied and cannot be undone.

Important Note: Currently tipping scans are discouraged since neither CASA or AIPS can apply the results of the tipping scans to the data.

Tipping scans may be needed if you are concerned about calibrating the absolute flux density of your science target source(s). The atmosphere absorbs some of the radiation, and the fraction of the absorbed radiation depends on the opacity, the transparency of the atmosphere. It is mainly dependent on the content of water vapor between the target source and the antenna(s), and can be derived from a series of system temperature measurements at various elevations. An observer would redetermine the opacity on the time scale in which significant changes are expected, i.e., the time scale in which the water vapor content of the atmosphere above the telescopes changes. This is a strong function of baseline length and actual weather and no real guideline on time scales is available.

Tipping scans are performed toward an AZ direction close to your sources at about the observing frequency. The scan samples elevations between about 20 and 60 degrees for a system temperature and can be directed from top to bottom (down) or from bottom to top (up). When you select an AZ for your tipping scan, be aware that shadowing may occur, especially in C and D-configurations. Avoid the AZ directions of the arms, i.e., avoid measuring tips close to the AZs of -5, 56, 115, 175, 236, 295, 355, and 416 degrees.

Setting Up a TIP Scan

To utilize the tipping scan mode, select Tipping from the drop-down menu under the scan mode section of a scan (Figure 4.22). Tipping scans are set up using one of your resources and probably are best done with the widest bandwidth available; make a new resource if you need it. You do not need a physical source. The total duration (LST) of a scan should include the required on source time of 1min 50sec for a tip scan to complete (in one direction, up or down). At the bottom of the page you will have to set the AZ and direction; do not forget this as otherwise you will be slewing to the default AZ of 0d (North) and may hit an antenna wrap constraint/limit. Hitting an antenna wrap limit, can consume half an hour of your observing time to return to your science observing, so always set the AZ.

Figure 4.22: Example of a Tipping scan.

You may place any number of tipping scans anywhere in your schedule as you feel fit to monitor the opacity during your observations, although you may want to do this close to your block(s) of high frequency observations. The first scan following the setup scan(s) may be a tipping scan, but as you don't know in what AZ the array starts, you want to allow for ample slewing time or anticipate a worst case scenario using the AZ starting conditions on the Reports tab.

If the tipping scan has not finished by the stop-time of this scan, the data will contain those elevation samples that were completed. If it has completed the tip before the stop-time, it simply will continue with the next scan until the regular stop-time for that scan — this scan may be used to buffer the difference, e.g., absorb the extra time on your bandpass calibrator.

A tipping scan is for post-observation calibration and may or may not yield useful data for your project. The data is included in the observations and you need special switches to load the data in CASA or AIPS. A "tip" would allow you to determine the opacity of the atmosphere during the tipping scan (i.e., during your observation), and you can use that value to correct for the atmospheric absorption in your data.

The Solar (SOL) observing mode is only utilized for a solar target, that is a location near or on the solar disk. This section will focus on how to setup solar source(s) in the SCT and how to use the SOL scan mode in the OPT. For more details on solar observing and how to setup a solar SB in the OPT, refer to the Solar Observing section of the Observing Guide.

Create a Solar Source

Before creating the observation, the observer must first create solar source(s) in the SCT. Since the Proposal Submission Tool (PST) cannot anticipate the solar target of interest, the observer’s source will not appear in any existing catalog and the observer must create one under the SCT.

To add a source to the catalog, click on FILE → CREATE NEW → SOURCE (see Figure 4.24).

Figure 4.24: Screen cap of a new source in the SCT.

Under the Source Positions field click on the pull-down menu to the right of Position Type and select Solar System Body with Uploaded Ephemeris (see Figure 4.25). For more information regarding the ephemeris, refer to the Solar Observing section of the Observing Guide.

Figure 4.25: Screen cap of new source position type Solar System Body with Uploaded Ephemeris.

Click on Browse to select a solar target ephemeris and then click Import. Note that the ephemeris must be geocentric and must be in JPL format. Enter a Name for the ephemeris target and, if necessary, repeat the process to enter additional targets into the catalog.

Setting Up a Solar Scan

Now that you have created the solar source(s) for the observation and assuming the resource has also been created in the RCT per the approved proposal. The next step is to create a solar observing scan in the SB.

To utilize the solar scan mode, select Solar from the drop-down menu under the scan mode section of a scan (Figure 4.26). This enables an additional pull-down menu at the bottom of the page under Mode Dependent Settings. The table below describes the parameters in detail and which to utilize.

Figure 4.26: Example of a Solar scan mode.

The VLA supports two different types of pulsar observing:

• Phase-binned imaging mode, in which visibility data are averaged into a number of bins corresponding to different rotational phases of the pulsar.  This allows, for example, separate imaging of on-pulse and off-pulse portions of the data.
• Phased-array pulsar mode (also called YUPPI mode), in which antenna data streams are coherently summed in order to produce a high-time-resolution, single-pixel data set.  The data can optionally be folded in real-time using a pulse period ephemeris.

Either mode can be configured via the Pulsar tab when creating an instrument configuration in the RCT.  For modes that require one, a Tempo-compatible period ephemeris file should be attached to each source in the SCT.  Additional information about setting up these modes can be found in the pulsar section of the Observing Guide, and in the OSS pulsar section.  If you have questions not addressed in these documents, please contact us through the NRAO Science Helpdesk.

The 27 VLA antennas can be used in subsets, to observe several independent programs simultaneously. Typical uses are to observe different sources that are bright enough that not all antennas are needed for the full sensitivity, or observing the same source at different observing bands at exactly the same time.

Subarray Guidelines

When preparing for subarray observing, all procedures, advice, and restrictions to create Scheduling Blocks (SBs) should be followed, as described in the previous sections of the manual. Any source and most resources available in the SCT or RCT catalogs, respectively, can be used in creating scans for the subarray. There are no restrictions in sources, scan intervals, scan timing, pointing scans, etc., between the subarrays other than the general limits, including restrictions for General Observing programs (i.e., up to three independent subarrays using standard 8-bit and 3-bit continuum setups and no special modes such as pulsar, OTF, etc., as that would make it a (Resident) Shared Risk program). There are restrictions in the division of antennas over the subarrays; in particular three subarrays of nine antennas each is not allowed. Further details on using subarrays can be found in the VLA Observing Guide and the Observational Status Summary (OSS).

Summary

• Subarray observing should be proposed for and be approved by the TAC.
• Subarray observing is General Observing and has restrictions accordingly; e.g., only standard observing with 8-bit continuum resources. Other modes or more than 3 arrays are in the Shared Risk categories.
• A division in three subarrays is typical one of 10, 9 and 8 antennas on the array; a division in two usually has 14 and 13 antennas per subarray allocated. Typical subarray antenna distributions over the whole array are homogeneous, random, or the inner antennas for the higher frequencies in a subarray and the outer for the lower frequency bands in another subarray. The latter is done to attempt to get a similar angular resolution between the observing frequencies.
• Subarrays cannot be generated with text-file uploads of SBs. However, if the Subarray Loop structure is in place, text-file scan lists can be imported in each subarray Loop separately (File → Import Scans...).

We are working on creating the three Reports separately for each Subarray Loop in an update of the OPT, but for now only the Schedule Summary Report (located at the bottom of the Reports tab of an SB) shows independent information per Subarray Loop.

How to Create Subarrays

Currently, creating a subarray schedule in the OPT is implemented by the use of scan loops with different scans (e.g., different sources, or the same sources with differing resources), where the loops are executed at the same time with different subsets of antennas. This is not the optimum implementation and we are working on a better scheme for the future. For the moment please follow these instructions:

• In the Program Block (PB) created for the project and approved for subarray observing, create a Scheduling Block (SB) if it is not already present. The details such as LST start range, weather conditions, count, etc., entered for this SB will apply to all of the subarrays created within the SB.
• To create a subarray in the SB, select the SB and make sure the SB is empty, i.e., no scans, then, select File → Create New → Subarray.
• To create additional subarrays within the same SB, select the newly created Subarray Loop and then select File → Create New → Subarray.  Up to three subarrays can be created for General Observing.
• Divide the array into subarrays, per Subarry Loop (see Figure 4.27).
• Tick the boxes of stations that should make up the subarray configuration.
• Already used antennas should not be selectable. Note that the numbering is from the center of the array (#1) out to the furthest one on the arm (#9), where N, E, and W stand for the North, East, and West arms, respectively. The allocated array configuration is specified in the PB; the actual antenna pad numbers scale with the principal array configurations (Any, A, B, C, D) but the order specified in the Subarray Loop is in which order the antennas are allocated on each arm per subarray.

Figure 4.27: Example of three subarrays under an SB.

• Create your Scan Lists in each of your subarrays.
  • Scan loops are allowed. Copying scans from one subarray to the next should work as usual, etc. However, do not use the tick-box Keep Previous Configuration under the Instrument Configuration for the first scan resource in your subarray. Make sure the total time per Subarray Loop adds to (almost) the same time, as otherwise the shorter subarrays will be idling while the longest has to finish (and which total time will count toward your allocated time).
• Once each Subarray Loop contains valid scans, the Reports page of the SB can be used to generate resource, source, and scan summaries. However, as a WARNING at this time, the Reports summaries include all resources and accumulated time on source over the subarrays. That is, all resources used in any Subarray Loop is listed in a single Resource Summary table. Also, if a source/resource/intent combination appears in more than one Subarray Loop, the total number of scans and total accumulated time is listed in a single Source Summary Table. However, when unfolding the loops in the Scan Listing table, they do list the scan listing per Subarray Loop correctly in the loops, i.e., each loop starts at the same LST to show the scan timings, elevations, etc., of the scans in each subarray. Check the end times of each of the loops to make sure they do not differ too much.
• Validate and submit the SB when done.

Disclaimer: Non-standard eLWA observing is currently listed as RSRO (Resident-Shared Risk Observing) and not GO (General Observing). A fixed version of this mode is available as Shared Risk Observing as of semester 2022A and available from the list of default setups, labeled as "eLWA (from semester 22A)". If you have questions regarding eLWA observing, please contact us through the NRAO Science Helpdesk.

VDIF stands for VLBI Data Interchange Format and was introduced in 2009 to standardize data transfer and storage for Very Long Baseline Interferometry (VLBI) observations. The expanded Long Wavelength Array (VLA + LWA = eLWA) observing mode uses this format to record a VDIF data stream for each VLA antenna. The VDIF files are then combined with raw voltage beam data from the New Mexico Long Wavelength Array stations and are correlated offline using a software correlator after the observation is completed.

The Fundamental Steps for Creating an eLWA SB

Step 1: Create VDIF Instrument Setup

First, go to Instrument Configurations (RCT), select the resource catalog you wish to create a new resource, and select FILE → CREATE NEW → 8-BIT INSTRUMENT CONFIGURATION.

1. Basics tab: Give the resource a name, e.g., VDIF 8 MHz, then select the receiver band 4 (54.0 - 86.0 MHz). The correlator integration time in this case is not relevant, just specify 5.0 seconds.
2. Basebands tab: Set the center frequencies to 536.0 MHz.
3. Subbands tab: Add one subband, select 8 MHz bandwidth, put BB_center-460.0 MHz to set central frequency to 76 MHz, and then set polarization to Dual and Recirculation to 1x.  
4. Special Modes tab: Select the VDIF tab and select Yes for Recording.
5. Validate tab: Make sure the summary looks like Figure 4.28 shown below and no validation errors are reported.

Figure 4.28: Validation page in the Resource Catalog Tool (RCT) of a valid VDIF resource.

Step 2: Enable scan for VDIF recording

When creating a new scan, select the above created VDIF instrument configuration. In order to enable VDIF recording, you also need to set the checkbox for VDIF Recording within the scan and it is also strongly recommended to disable the 10 Hz switched power, by selecting the Disable checkbox. Your scan should look similar to the example in Figure 4.29 below.

Figure 4.29: Example of a scan using VDIF recording.

Important Note: If you plan to trigger resetting requantizers using a setup scan and the VDIF recording instrument configuration, then 10 HZ SWITCHED POWER must be unchecked, to have switched power enabled, and VDIF RECORDING should be disabled by leaving it unchecked.