Subarray Observing

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.