In order to select guide stars suitable for closed-loop offsetting, the positions of the target and a nearby reference star of accurately known relative position must be entered in a catalogue. This is a simple text file, and the allowed formats are described in detail in Section 7.1. In most circumstances, only the name, RA, Dec and equinox of each object need to be specified. For example, in order to observe a target 3C789 with a reference 3C789REF, the catalogue file (BLACK.CAT) would look like:
3C789 20 13 14.156 45 45 45.45 J2000 3C789REF 20 13 16.020 45 44 06.34 J2000To select a suitable guide star in order to observe with the INT at PA 90:
$GSS GSS>CONFIG INT CASS GSS>ROT 90 GSS>CATA BLACK.CAT GSS>BLIND 3C789REF 3C789 GSS>EXIT $TYPE OUTPUT.GSThe output gives the probe coordinates appropriate for the reference star and the offset required to acquire the target (this will be applied automatically by the INT TCS command AUTOFFSET described in the INT Manual, although beware the fact that the target and reference must have coordinates at the same equinox in this case).