##
...Test if a WCS is a Celestial Coordinate System

The world coordinate system (WCS) currently associated with an image may often be a celestial coordinate system, but this need not necessarily be the case. For instance, instead of right ascension and declination, an image might have a WCS with axes representing wavelength and slit position, or maybe just plain old pixels.

If you have obtained a WCS calibration for an image, as in §3.4, in the form of a pointer “wcsinfo” to a FrameSetFrameSet, then you may determine if the current coordinate system is a celestial one or not, as follows:

12#12

This will set “issky” to 1 if the WCS is a celestial coordinate system, and to zero otherwise.