CASPIR Aperture definition
The telescope pointing coordinates apply to one particular spot
in the focal plane. This is the "current aperture".
The field in an Alt-Az telescope rotates and the current aperture is the
spot around which the system de-rotates the field. Tracking drifts and large
pointing errors will occur if this is not the right spot! Hence the importance
of doing both aperture and pointing calibrations.
At least one aperture is usually defined and given a name
(for CASPIR usually "a" and located in the centre of
the detector, pixel (128,128)), by the interactive procedure
calibrate/aperture (or aperture/calibrate , exactly the same).
Aperture 0 (zero) is predefined as the optical axis of the telescope.
Usually the centre of the detector is slightly off that axis.
The current aperture is then selected by the command
aperture aperture_name .
It is usually enough to do one aperture calibration at the beginning
of a run, as the calibration will only be disturbed when the instrument
is taken off the telescope, or is cleared on an observer name change.
It may be useful to write down the aperture calibration information,
as this can be re-entered with aperture/define (refer manual,
or type help aperture ) in case it gets accidentally deleted.
That saves the hasle of doing the interactive calibration again.
If after some strange event, like a system crash, no stars can be found,
one should not only look at the pointing coefficients, but also make sure
a valid aperture is selected, with a reasonable position. The telescope will
make stars appear in the current aperture which, with a rogue definition,
could be way off the detector. Check this with aperture/show .
(To be on the safe side, select aperture 0 .)