If we print out the FITS header cards describing the SkyFrame we wrote
earlier (), we should obtain something
like the following:
COMMENT AST ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ AST COMMENT AST Beginning of AST data for SkyFrame object AST COMMENT AST ................................................................ AST BEGAST_A= 'SkyFrame' / Description of celestial coordinate system NAXES_A = 2 / Number of coordinate axes AX1_A = ' ' / Axis number 1 BEGAST_B= 'SkyAxis ' / Celestial coordinate axis ENDAST_A= 'SkyAxis ' / End of object definition AX2_A = ' ' / Axis number 2 BEGAST_C= 'SkyAxis ' / Celestial coordinate axis ENDAST_B= 'SkyAxis ' / End of object definition ISA_A = 'Frame ' / Coordinate system description SYSTEM_A= 'FK4-NO-E' / Celestial coordinate system type EPOCH_A = 1958.0 / Besselian epoch of observation ENDAST_C= 'SkyFrame' / End of object definition COMMENT AST ................................................................ AST COMMENT AST End of AST data for SkyFrame object AST COMMENT AST ---------------------------------------------------------------- AST
As you can see, this resembles the information that would be written
to a basic Channel to describe the same SkyFrame
(), except that it has been formatted
into 80-character header cards according to FITS conventions.
There are also a number of other differences worth noting:
This last point is worth further comment and is necessary because the FITS standard only allows for certain keywords (such as COMMENT and HISTORY) to appear more than once. AST_WRITE therefore appends an arbitrary sequence of two characters to each new keyword it generates in order to ensure that it does not duplicate any already present in the FitsChan.
The main risk from not following this convention is that some software might ignore (say) all but the last occurrence of a keyword before passing the FITS headers on. Such an event is unlikely, but would obviously destroy the information present, so AST_WRITE enforces the uniqueness of the keywords it uses. The extra characters added are ignored when the information is read back.
As with a basic Channel, you can also suppress the comments produced
in a FitsChan by setting the boolean (integer) Comment attribute to
zero (). However, FITS headers are
traditionally generously commented, so this is not recommended.
AST A Library for Handling World Coordinate Systems in Astronomy