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 , in the form of a pointer ``wcsinfo'' to a FrameSet, then you may determine if the current coordinate system is a celestial one or not, as follows:
AstFrame *frame; int issky; ... /* Obtain a pointer to the current Frame and determine if it is a SkyFrame. */ frame = astGetFrame( wcsinfo, AST__CURRENT ); issky = astIsASkyFrame( frame ); frame = astAnnul( frame );
This will set ``issky'' to 1 if the WCS is a celestial coordinate
system, and to zero otherwise.
AST A Library for Handling World Coordinate Systems in Astronomy