Astrometry header standard for FITS?
Andy Puckett wrote on Dec 10, 2007
I am looking for some help understanding the FITS standard for encoding an astrometric solution in the header. Specifically, is the cos(dec) correction for RA supposed to be encoded in the CDm_n/CDELTn keywords, or is it understood that it will be applied by the FITS reader after the fact?
For example, if you had an image with 0.28"/pixel at a declination of 30 degrees, and keyword headers:
CTYPE1 = 'RA---TAN '
CDELT2 = 0.000078
would you expect to see:
CDELT1 = -0.000078
or:
CDELT1 = -0.000090
?
Also, do you have a feel for whether there are groups out there that are not following the standard? If there are, then how is the FITS reader software supposed to determine whether or not to apply the correction?
For example, if you had an image with 0.28"/pixel at a declination of 30 degrees, and keyword headers:
CTYPE1 = 'RA---TAN '
CDELT2 = 0.000078
would you expect to see:
CDELT1 = -0.000078
or:
CDELT1 = -0.000090
?
Also, do you have a feel for whether there are groups out there that are not following the standard? If there are, then how is the FITS reader software supposed to determine whether or not to apply the correction?
Mike Fitzpatrick wrote on Dec 10, 2007
See Frank's reply in http://iraf.net/phpBB2/viewtopic.php?t=86862
Last post on Dec 10, 2007