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Subarray Observing

by Gustaaf Van Moorsel last modified May 22, 2017 by Vivek Dhawan

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.

When preparing for subarray observing, all procedures, advice, and restrictions to create Scheduling Blocks should be followed, as described in the previous sections of the manual. Any source or resource available in the SCT or RCT catalogs, respectively, can be used in creating scans for the subarray. There are no restrictions in sources, resources, scan intervals, scan timing, pointing scans, etc., between the subarrays other than the general limits, including restrictions for General Observing programs (i.e., for semester 18A and 18B up to three independent subarrays using standard 8-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 Guide to VLA Observations and the Observational Status Summary (OSS).

 

Making Subarrays in the OPT

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 created for the project and approved for subarray observing, create a Scheduling Block if it is not already present and name the Scheduling Block. The details such as LST start range, weather conditions, count, etc., entered for this Scheduling Block will apply to all the subarray observations.
  • Click on the name of the newly created Scheduling Block in the tree to the left if it is not already active (highlighted). Make sure it is empty, i.e., remove any (default) scan already in the Scheduling Block. Use File-Create New-Subarray to create a new subarray in the Scheduling Block and name the Subarray Loop. Repeat, now or later, for one or two more subarrays (up to three total for General Observing). You must have selected a Subarray Loop (not the Scheduling Block) to create additional ones.
  • Divide the array into subarrays by, per Subarray Loop, ticking 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 arm respectively. The valid principal array configuration (Any, A, B, etc) is specified in the Program Block; the actual antenna pad numbers scale with the principal array configuration but the order specified in the Subarray Loop is in which order the antennas are allocated on each arm per subarray.
  • Create your Scan Lists in each of your subarray. Loops of scans are allowed. Copying scans from one subarray to the next should work as usual, etc. Do not use the tick-box "Keep Previous Conf." for the first scan resource in your subarray. Make sure that 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 Scheduling Block 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 correcly 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 the Scheduling Block and submit when done.

 

Notes

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 Scheduling Blocks (File-Import Scheduling Block). 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 a next update of the OPT, but for now only the Scan Listing summary shows independent information per Subarray Loop.