Subarray Observing

The VLA can be split up in subarrays. That is, some of the 27 antennas and corresponding baselines can be ordered to do a completely different and independent program than other antennas. This may be the case when an observer has asked to divide up the array for a single project to observe a source simultaneously in multiple bands or to observe multiple different sources simultaneously that do not need the full array, or when one antenna is split off from the main array for inclusion in a VLBI array by another user (though not currently offered).

 

The current restrictions for observing with subarrays are:

  • The use of subarrays has to be requested in the proposal and approved by the TAC.
  • Up to 3 subarrays may be used. Note that the array cannot be divided up in three equal subarrays of nine antennas. For instance, three subarrays may be obtained by having 10 antennas in subarray 1, 9 antennas in subarray 2, and 8 antennas in subarray 3. For more information and for other possibilities see the subarray configuration details in the VLA OSS.
  • Until further notice, only 8-bit default NRAO wideband continuum frequency setups may be used for standard interferometric  observing (no phasing, binning, etc). Other modes may be  offered as RSRO in the call for proposals.
  • The division of pointing directions, frequency bands,  polarization products and integration times over subarrays is  unrestricted. Some of this freedom may be limited for OTF modes  and pointing scans.
  • For every subarray a separate scheduling block of (near-)equal length should be submitted through the OPT, with the same starting conditions and weather constraints, and appropriate comments to the operator; see example comments below.
  • The sum of data rates and other restrictions and guidelines for standard (single subarray) observations must be taken into account.
  • Note that subarrays are started in sequence with some time in  between, typically a minute for each additional subarray. If  absolute simultaneous scans are required, make sure to allow  for this in the individual schedules (and in the comments).

If in doubt please contact the NRAO user helpdesk.  Soon, the OPT will have the possibility to better specify subarray observations by grouping the scheduling blocks in a single entity. Please stay tuned.

Example comments to the operator:

In the "Comments to the Operator" field of the scheduling blocks, the operator will be looking for the following information:

  • This SB is part of a subarray observation
  • The SB ID numbers of the entire observation
  • The division of antennas over the subarrays
  • The order and interval of starting the individual subarrays
  • If less than 27 antennas are available, how to adjust the above

Please limit the constraints to the absolute necessary ones, as the more constrained the configuration and requirements, the less flexible the observing run, and thus less likely that the observation will fit into the observing queue.

Example 1: most simple case

This SB, ID 13096420, is one of two subarrays; the other subarray SB ID is 13096422. Please divide the antennas in two equal subarrays (of 12 antennas or more) roughly spread evenly over each antenna arm. The SB starting order is unimportant.

Example 2: little more constraints

SB IDs 11088375, 11088377 (this one) and 11088379 are part of a 3-subarray observation. Please divide the array up using 8 antennas at the outer parts of the arm observing in L-band (11088375), 10 antennas using Ka-band (11088379) in the inner part of the array, and 9 antennas using X-band (11088377) in the central part of the configuration. If less than 27 antennas are available, strip one or two from the X-band subarray before stripping one from the L-band subarray (11088375). Please start the subarray in Ka-band (11088379) first and L-band (11088375) last.

Example 3: very constrained and not recommended (reduced chances of observing!)

Three-subarray observation. Please use the following configuration:
* sub1, 10 antennas = N01, N03, N05, N09, W02, W06, W09, E02, E06, E09, SBID #12043017, Q-band, start first
* sub2, 9 antennas = N02, N06, N08, W03, W05, W08, E03, E05, E08, SBID #12043019, K-band, start 60s after the start of Q-band
* sub3, 8 antennas = N04, N07, W01, W04, W07, E01, E04, E07, SBID #12043021, X-band, start 120s after the start of Q-band.
If an antenna for sub3 is not available, please put N03 of sub1 in sub3.

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