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Choice of an initial search area

The object here is to choose an area of sky, bounded by curves of constant Right ascension and Declination, which just includes the guide field. The outer boundaries of the guide fields are either circular or rectangular in the tangent plane. In the former case, the procedure in the CHART subroutine SELECT should be followed. Except for targets close to the Poles, the ranges of RA and Dec are given by: where R is the radius of the guide field and f is the focal length. This formula fails for the case where or ( i.e. the guide field includes one of the Poles), in which case all stars with (in the North) or (in the South) should be selected. For rectangular guide fields, the RA and Dec of each corner should be derived from its tangent-plane position using the formulae: and the widest range in each coordinate should be used in the initial search. The SLALIB routine DS2TP does the necessary conversions. A similar caveat applies to guide fields around the Poles, for which the selection should be made on Declination alone, as for circular guide fields.