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 (except for targets near the Poles),
the ranges of RA and Dec are given by:

$\Delta \alpha =arctan(R/fcos\delta $_{0})

$\Delta \delta =\; R/f$

where R is the radius of the guide field and f is the focal length. This formula fails for the case where $\delta $For rectangular guide fields, the RA and Dec of each corner are derived from its tangent-plane position using the formulae:

$tan(\alpha -\; \alpha $_{0}) =
ξ_{0} / (f - η_{0})

$tan\delta =\; \{\eta +\; ftan\delta $_{0}_{0})^{2} + ξ^{2}^{2}δ_{0}]^{1/2}}

Mon Mar 1 17:52:09 GMT 1999