The outer edge of the field is a circle of radius 16 arcmin (defined by the
filter wheel). The guide probe is on an xy stage and can
cover all of this area except for a small part outside its -y range and a
cut-out at large x and y. The inner edge of the field is elliptical with a
semi-major axis of 14 arcmin along x and a semi-minor axis of 10 arcmin
along y. The ellipse is produced because the autoguider is fed
by a flat mirror with a circular hole inclined at 
to the optical
axis. The additional conditions are:
and the area with 
and 
is also excluded.