[fitsbits] FIT Keyword data dictionaries

[William Pence]

Peter Bunclark wrote:
> 
> The FITS standard states angles should be in degrees!

Actually, the FITS Standard is more qualified in its recommendation on units
(in section 5.3): "For angular measures given as floating point values and
specified with reserved keywords, degrees are the recommended units."  This
wording was carefully chosen to make it legal to specify angular quantities
as strings in 'dd:mm:ss' or 'HH:MM:SS' format for non-reserved keywords.

In particular the 'RA' and 'DEC' keywords have often been used in the past
with a string value such as

   RA      = '12:30:56.5'
   DEC     = '-63:56:45.67'

For this reason, the definition of these 2 keywords in the FCG common
keyword data dictionary (available at http://legacy.gsfc.nasa.gov/fcg)
allows the values to be given either as a character string (as in the above
examples) or as a floating point number in units of decimal degrees.  This
seemed like the simplest way to accommodate the historical usage of these 2
keywords.  All the other keywords in the data dictionary which have angular
values are required to be in units of decimal degrees.

-Bill Pence
____________________________________________________________________
Dr. William Pence                          pence at tetra.gsfc.nasa.gov
NASA/GSFC Code 662         HEASARC         +1-301-286-4599 (voice)     
Greenbelt MD 20771                         +1-301-286-1684 (fax)