The best way to determine the correct telescope focus is to use
the ICL FOCUSIMAGE procedure (see Section 6.4).
FOCUSIMAGE generates a single CCD frame with a number of
equal length exposures of a star(s), with the telescope
position and focus stepped between exposures (the first step is
double so that the stepping direction can be determined). The resulting
image can then be examined
(e.g. with IRAF IMEXAMINE) to find the best focus. Once the
best focus <position> has been determined, set the
telescope focus by typing:
FOCUS <position> at the ICL interface.
The telescope focus will be automatically compensated for temperature changes.
To enable automatic compensation of the telescope focus for
different filter thicknesses, type :
PSETFOC ENABLE at the ICL interface.
Similarly, PSETGDFOC ENABLE allows automatic adjustment of the Autoguider focus to compensate for different Autoguider filter thicknesses.
The database with the names and focus offsets of the filters in the main filter wheel can be updated using the ICL PCHANGE procedure. This should be done before PSETFOC is enabled. See Section B.7.2 for how to calculate focus offsets. It is a good idea to do a FOCUSIMAGE with all of the filters that will be used to check the calculated focus offsets, and adjust them if necessary. See Section 6.1 for more details of the above procedures.