RE: FITS celestial coordinate keywords.

[pence@tetra.gsfc.nasa.gov (William Pence)]

Barry Schlesinger wrote:

>> RA_OBJ    /  position of the object specified by the OBJECT keyword (degrees)
>> DEC_OBJ   /
>
>> RA_NOM   / nominal pointed position of the observation (degrees)
>> DEC_NOM
>
>>...[other proposed keywords]
>
>How would these conventions be used together with the World 
>Coordinates projection conventions, (e. g. RA---TAN)?

The new keywords have only an indirect connection to the FITS WCS keywords.
The RA_OBJ and DEC_OBJ keywords just give the coordinates in degrees
of the object specified in the OBJECT keyword.  The RA_NOM, DEC_NOM
keyword give the planned, or nominal pointing of the observation.
The RA_PNT, DEC_PNT keywords give the actual (as well as can be 
determined) pointed position of the optical axis during the observation.
In the case where the FITS primary array is a celestial image (in RA and DEC)
then RA_PNT may well be equal to CRVAL1 and DEC_PNT may well be equal 
to CRVAL2 (if the reference pixel is on the optical axis), but this
is not required.  The WCS keywords should always be used to
determine positions in an image.

-Bill Pence