[iaufwg] FITS conventions

[Francois Ochsenbein]

Dear Bill & IAUFWG members,

About the FITS conventions which should be clarified, there is
the long-standing convention about color images which was discussed
several times, but which never reached an agreement. FITS can be
discarded (e.g. by amateur astronomers) as an interesting format 
just for this lack of a definition, which I feel is a pity.

The two approaches for storing RGB images in FITS files are, either to
store one color as the main 2D matrix and other colors in FITS extensions,
or to use a 3D 'matrix' with a convention specifying which 'axis' contains
the color information (e.g. via CTYPE='RGB' as proposed many years ago). 
I know that several discussions occured about these alternatives and the 
usage of 'pseudo-dimensions', but a recommendation of the FITS authorities 
on how to write color images in FITS would avoid to see a proliferation of
contradictory conventions about this issue.

One immediate help could be to add to the FITS documents a list of
the "official" CTYPEs . This list should contain, in addition to 
the WCS conventions, at least the "STOKES" convention which is 
widely used, and the "RGB" convention which was discussed many times.

Does this sound impossible ?

Cheers, francois
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Francois Ochsenbein       ------       Observatoire Astronomique de Strasbourg
   11, rue de l'Universite F-67000 STRASBOURG       Phone: +33-(0)390 24 24 29
Email: francois at astro.u-strasbg.fr   (France)         Fax: +33-(0)390 24 24 17
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