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
, 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 SchedBlock. 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 a real case there are other considerations. Here we bring in the following two conditions:
In terms of hour angles, splitting is performed only in cases where the elevation of the target is low, that is, difference of celestial declination of the target and observatory latitude is big, and also very high resolution is desired. In this case the "Y" configuration is needed which will not give good uv coverage for a low elevation. The threshold for elevation used here is 35 degrees and the one for resolution is 0.1 arcsec; consequently the observation of which desired resolution is less than 0.1 arcsec and elevation is lower than 35 degrees is split into two SchedBlocks, and they have 0, 12 and 12, 24 as minimum and maximum allowed hour angle, respectively.
The ALMA OT Team, 2012 May 25