Although there is no strict need for the observer to select guide stars in advance, since locating them on the finder field is simple and quick, it is desirable to measure guide star coordinates before going to the telescope, and at Prime Focus in particular it is worth checking that there is a suitable guide star in the field accessible to the probe, especially if there are constraints on the exact position angle of the CCD chip on the sky.
A scale diagram of the Prime Focus detector and guide-probe fields is shown in
figure 
; the diagram for Cassegrain is show in
figure . In order to check that there is a suitable guide star,
place a scaled version of the overlay
on the POSS with the chip outline centred on the object to be observed.
Rotate the
overlay to your chosen position angle. If you do not care, use 0 at
Cassegrain, and 180 at Prime,
because this avoids unnecessary loss of liquid nitrogen at large zenith
distances in the North. Check that there
is a star with 714 in the probe field. If there is not, try
again with the overlay rotated by 180 (if you care about the
orientation) or any angle (if you don't, but remember that cardinal
points give better displays). Make a note of the approximate PA to be used; at
the telescope you can locate the star on the Finder TV, or, if you happen to
know its coordinates, then these can be entered at the User Interface console.