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Algorithm Used for Splitting SBs

The algorithm used to split the requested observation into one ore more SBs considers two requirements: the need to cover the desired range of angular scales, and the need to ensure full uv-coverage. The ALMA configurations are designed so that in most cases good uv-coverage is obtained in one go. They generally also provide a wide range of baselines, meaning only a few configurations are needed to cover most angular ranges desired.

The angular scale range which an array configuration covers is determined by the ratio of the maximum and minimum baselines of the configuration. Values of this ratio, $N$ , are different among each array configurations; the value $N=50$ is used as a representative value at present.

Let SARG = Smallest Angular Resolution Goal and LARG = Largest Angular Resolution Goal. If $N * SARG < LARG$ , then it is impossible to cover the angular range by a single array configuration. In such a case, the observation is split into more than one SB. The ideal split would be an SB with a desired resolution goal of SARG, then one with a goal of N*SARG, and so on.

However, in reality there are other considerations. Here we bring in the following two conditions:

  1. We aim for SBs with angular resolution ranges that overlap by 10%. This makes the split SARG, (1-0.1)*N*SARG, and so on.
  2. For scheduling we allow a 50% margin which specifies the range of acceptable angular resolutions. This makes the split (0.8*SARG, 1.2*SARG), (36*SARGS, 54*SARGS), and so on.

In terms of hour angle, splitting is only performed when the elevation of the target is low and the resolution required is sufficiently high that one of the "Y" configurations is needed; this combination will not give good uv-coverage. The threshold for elevation is 35 degrees and that for resolution is 0.1 arcsec; consequently observations for which the desired resolution is less than 0.1 arcsec and the elevation lower than 35 degrees are split into two SBs with 0, 12 and 12, 24 as the minimum and maximum allowed hour angles.


next up previous contents index
Next: Phase 2 Validation and Submission Up: Generate Scheduling Blocks Previous: Scheduling Block Restrictions   Contents   Index
The ALMA OT Team, 2011 May 20