Unsigned integer support in NOST 100-1.2

[William Pence]

Phil Hodge wrote:

> This is trivial to implement, yes, but it's not transparent to many
> people who would read the header.  It's rather esoteric, in fact.
> I think that FITS should support unsigned 16-bit and 32-bit integers
> using notation (e.g. a DATATYPE keyword) that would be clear to an
> intelligent person who is not necessarily an expert on FITS.

The FITS standard allows optional characters following the datatype
character in the TFORMn keyword, so one could adopt a convention
something like:

TFORM1  = '1IU'               / datatype is unsigned 16-bit integer
TZERO1  =               32768

or

TFORM1  = '1I(unsigned)'      / datatype is unsigned 16-bit integer
TZERO1  =               32768

This doesn't require any change to the FITS standard, and the TZEROn
keyword is still required, but at least it would be clearer to  people
reading the header that this is an unsigned integer column. 

____________________________________________________________________
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)