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Special behaviour of FITS headers

A complication that we have glossed over so far is the fact that certain FITS header items can occur more than once within an image. Strictly, this is limited to the keywords COMMENT, HISTORY and ' ' (blank) which are often used to construct multi-line descriptions of the contents or history of a FITS file. To handle this, various of the HDR subroutines are ``FITS occurrence aware''. For instance, the fourth argument of the HDR_IN subroutine specifies the occurrence of the item to be read, and HDR_OUT will append (rather than overwriting) items with these special names.

FITS items can also have a comment associated with them (to be read by the recipient when a FITS file is sent to another astronomical institution). The comment string is given by the fourth argument of the HDR_OUT subroutine, but this facility isn't available for non-FITS header sources where this argument will simply be ignored.



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IMG Simple Image Data Access
Starlink User Note 160
P.W. Draper
R.F. Warren-Smith
3 March 2003
E-mail:P.W.Draper@durham.ac.uk

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