Reading WCS Information Stored in Other Forms

When a KAPPA application requires WCS information, it looks first in the WCS component of the NDF. If no WCS component is defined within the NDF, then it will attempt to obtain WCS information from two other locations, in the following order:

1. If an IRAS90 astrometry structure (see SUN/163) is present within the NDF, then WCS information will be read from it, and added to the FrameSet  holding the GRID, FRACTION, PIXEL, and AXIS Frames. IRAS90 astrometry structures have been used by several applications packages in the past for storing astrometry information. KAPPA contains the SETSKY application which can be used to create such a structure, either by supplying the required numerical parameters (pixel size, image orientation, etc.), or by doing a least-squares fit to a set of pixel co-ordinates with corresponding celestial co-ordinates.

2. If no IRAS90 astrometry structure can be found, an attempt is made to read WCS information from the FITS header cards in the FITS extension.

Note, KAPPA applications always write WCS information in the form of a standard WCS component. You should remember that astrometry information stored within an IRAS90 or FITS extension will not be corrected to take account of geometric manipulation produced by applications such as ROTATE, COMPAVE, etc. Use of such applications will render IRAS90 and FITS astrometry information incorrect. For this reason, applications always warn the user if astrometry information is being read from an IRAS90 or FITS extension. These extensions can be deleted if necessary, using the ERASE command. For instance:

   % erase m31.more.iras90
   % erase m31.more.fits

will erase the IRAS90 and FITS extensions from the NDF m31.