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...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 [*], 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.


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AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 211
R.F. Warren-Smith & D.S. Berry
24th May 2011
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2009 Science \& Technology Facilities Council