ON THE NECESSITY OF A MODERATE NUMBER OF SUBARRAYS FOR ALMA S. T. Myers (NRAO) 29 February 2000 ----------------------------------------------------------- ABSTRACT: For some time now, there has been debate about the actual number of simultaneous subarrays required for ALMA. From the system design point of view, arguments have been presented that 4 is sufficient. There is even a baseline design for 2 subarrays that has been floated. However, for some time now, scientific arguments have been given that compelling cases exist for having more than 4 simultaneous subarrays. I propose that a baseline of 8 subarrays would be a moderate design goal, even a reasonable design specification, given the probable uses of the ALMA array. Below I present a concrete example of a case using 8 subarrays, and discuss why this should be taken seriously in the current debates about the design. ----------------------------------------------------------- The following is a strawman subarray allocation table given as a concrete example of an Nsub = 8 case: #1 Antennas out for reconfiguration and baseline determination #2 Antenna(s) used for mm-wave VLBI #3 Antenna(s) undergoing receiver testing or warm-up/cool down #4 Main interferometric subarray #5 Main single-dish subarray #6 Secondary interferometric subarray at second frequency band #7 Secondary single-dish subarray at second frequency band #8 Secondary interferometric subarray for transient event monitoring Discussion: First, the engineering subarrays - #1 Antennas out for reconfiguration and baseline determination It is highly likely that at any given time there will be several antennas being moved or having just moved, and some number of antennas being used to anchor the new baselines into the array. #2 Antenna(s) used for mm-wave VLBI Several time a year (at least) one or more ALMA antennas will be used to perform mm-wave VLBI with the VLBA and other VLBI antennas. #3 Antenna(s) undergoing receiver testing or warm-up/cool down It is almost certain that at any given time there will be at least one 4K dewar out of commission at any given time, and probably more than one, especially during the early years of operation as we gain experience. Furthermore, we will likely be testing new receiver components and bands, and therefore a third engineering subarray is probably fairly common. Now, the observing subarrays - #4 Main interferometric subarray This is the primary observing array #5 Main single-dish subarray Many (most?) observations will require or desire simultaneous single-dish OTF scans to allow high-fidelity mosaicing. There are numerous documents out there on this. 'Nuff said. #6 Secondary interferometric subarray at second frequency band #7 Secondary single-dish subarray at second frequency band It is also possible (likely) that not all antennas will have receivers at all bands (especially the submm bands and especially during the early years of the array). For example, an inner ring might be outfitted with the highest-frequency receivers doing the primary observations (subarrays 4-5) and the outer antennas in the array be observing at some lower frequency band. THIS WAS DE RIGEUR DURING THE EARLY DAYS OF THE VLA Q-BAND INSTALLMENT! Finally, #8 Secondary interferometric subarray for transient event monitoring At any given time, there is a GRB to observe. Furthermore, there are a number of X-ray transients requiring observation. It is impossible to extrapolate from the current VLA situation, but it is absolutely clear that there will be the desire to commandeer a subarray for catching, following-up, and monitoring these interesting objects or ones like them. Furthermore, there are more mundane cases where the ability to pull out a small subarray for monitoring is called for. An example is time-delay monitoring of gravitational lenses. (It might be advantageous to monitor at 85GHz where variability is high, and there is the whole southern sky.) Although this may be an extreme case, it is absolutely clear that 2 subarrays is incapable of supporting even normal operations, and that even 4 subarrays is insufficient. This is particularly true in the early stages of the array construction, where there is most likely going to be a heterogenous mixture of receiver suites among the existing elements. Therefore, even a design where 2 initial subarrays can be expanded later on will seriously hamper operations and testing. I would like to reiterate that even 4 subarrays is insufficient. For example, a common mode might be #1 Antennas out for reconfiguration and baseline determination #2 Main interferometric subarray #3 Main single-dish subarray #4 Second interferometric subarray at second frequency band which uses a full 4 subarrays. There is no headroom in a 4-subarray design for a transient response subarray or VLBI. Conclusions: I propose that the design goal for ALMA be 8 simultaneous subarrays. There may be reasons why the actual specification be "at least 4 subarrays", though this will be insufficient for all but the most mundane operational modes and thus I strongly urge Nsub = 8 be adopted as the spec from the beginning. It is also greatly desirable that no undue limitation be imposed on the number of antennas allocatable to any given subarray, as this may be driven by either the scientific requirements of a given program, technical requirements of an engineering task, or the chance availability of receivers or configuration status. Although these requirements may have substantial budgetary impact, unnecessary frugality or conservatism at this point will likely hamper the full use of ALMA as the flagship radio-astronomical observatory of the following decade. There are compelling scientific and operational arguments in favor of having the capability of 8 simultaneous subarrays, and in strong opposition to having fewer than 4 subarrays. Although 4 subarrays may be seen as fulfilling the baseline requirements for "normal" operations, there are clearly cases of interest where more than 4 would be desired, if not necessary. Furthermore, this capability must be included in the current design, and cannot be left as a later upgrade, as it is most likely the early testing stages where a number of engineering subarrays will be used which will detract from the number available for scientific observations. Note that none of the arguments presented here are new, and have been expressed by other members of ALMA/MMA working groups, particularly Michael Rupen and Bryan Butler. We (the Imaging and Calibration group in particular) would like to see these concerns more clearly addressed in the discussions currently underway before crippling limitations are built into the rapidly evolving designs. Note that the above arguments do not even include considerations of a set of smaller antennas. In that case, it is likely to require a subarray of its own in some cases (where having heterogenous subarrays is not desired). The reader is referred to the scientific requirements most recently presented in the white paper "Astronomical Requirements for the Millimeter Array Correlator" by Rupen, Shepherd and Wright (1998).