[fitsbits] 64-bit integers

[William Pence]

I would like to reopen the discussion about whether 64-bit integer images
and 64-bit integer binary table columns should be supported in FITS files. 
This topic was previously discussed here back in July 1998 when Don Jennings
suggested that the NOST Panel should add support for 64-bit integers in its
revision of the FITS Standard.  The NOST panel ultimately decided that it
was not within its charter to make such a major change to the FITS format
and thus never issued an opinion one way or the other on the technical
merits of the suggestion.

The case for supporting 64-bit integers in FITS has gotten even stronger in
the 3 years since that previous discussion.  64-bit CPUs and operating
systems are now standard in the computer industry.  Also, as we enter the
"Virtual Observatory" era, it is important that FITS be able to support all
the types of data files that people (both observers and theoreticians) will
want to store and access.

It will be fairly easy to modify the FITS Standard itself to support 64-bit
integers: the allowed values for the BITPIX keyword can be extended to
BITPIX = 64 to signify 64-bit integers in a primary array or image
extension, and a new 'K' binary table format can be added (as in TFORMn =
'1K')  to support 64-bit integers in a binary table column.  It will of
course require quite a bit more effort to convert existing software systems
to support this new datatype once it is adopted.

Some existing FITS I/O libraries already support 64-bit integers. Tom
McGlynn's Java FITS library
(http://heasarc.gsfc.nasa.gov/docs/heasarc/fits/java/v0.9/) for example, has
always supported 64-bit integers because Java naturally supports this
datatype on all platforms.  Recently I also modified my CFITSIO library to
at least partially support 64-bit integers (but this is not yet available in
the publicly released version of CFITSIO).  A sample FITS data file
containing a 64-bit integer primary array and a binary table extension with
a 64-bit integer column is available at
ftp://heasarc.gsfc.nasa.gov/software/fitsio/test64bit.fit.  I would be
interested in hearing of any other software systems that can read this
non-standard FITS file.

The question I would like to raise is, should a proposal to support 64-bit
integers in FITS now be drafted and submitted to the 3 regional FITS
committees for their consideration?  Are there any compelling reasons not to
do this now?  

-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)