Subarrays

The VLA can simultaneously process five different observing programs. However, the subarrays are not all fully independent. If use of this capacity is contemplated, the following limitations must be understood and followed:

1. Each subarray uses a different observing file. (Strictly speaking, this is not a requirement, but is sensible.) The VLA operator must be told which file is to control which subarray, and which antennas are to be in each subarray. Antenna `shuffling', in which antennas are reassigned from one subarray to another after observing has begun, is strongly discouraged.
2. Only one integration time for the entire array can be defined. Unless told otherwise, this time is that assigned for subarray #1. All other requested integration times (which are given on the //DS card in the observing control file) are ignored.
3. All subarrays must be in continuum mode, or all must be in spectral line mode. Any spectral line/continuum mix will result in no data at all. This is true for both standard observing and for a pointing determination.
4. If an IF (i.e. A, B, C, or D) is used in more than one subarray, it must have the same bandwith in each. (This restriction applies only to spectral line observations).
5. There are only two sets of Fluke synthesizers (the final LOs in the frequency conversion system) - hence, only two subarrays can have completely independent frequency selection. If using two subarrays, notify the operator about your requirements for synthesizer setting - this selection is not made within the observing file. For three or more subarrays, a decision will have to be made on which subarrays will be `slaved'. This will constrain the frequencies used for bandwidths other than 50 MHz.

Note that tipping scans (used to measure the atmospheric opacity) can be done at any time by any number of subarrays. If any of the above restrictions confuse you (and we are sympathetic if they do), consult a VLA staff member, or talk to Ken Sowinski.

Single-antenna (or multiple-antenna) VLBI programs cause special problems. Such programs use one of the Fluke sets, leaving only one for the remaining four subarrays - these must then share that single Fluke set, or use the same values assigned to the VLBI run. Generally, VLBI Fluke settings are compatible with continuum observing. Fortunately, the correlator restrictions listed above (points 3 and 4) do NOT apply to the single-antenna VLBI subarrays.