FITS long integer support (was Re: [fitsbits] ADASS FITS BoF on Sunday)

[William Pence]

We should perhaps consider adding a short section to the FITS Standard to 
clarify the status of long (64-bit) integer support in FITS for the benefit 
of users.  Here is a first cut at the main points we might want to make. 
Any comments?


1. There is in principle no limit on the magnitude of integer keyword values 
in FITS headers and on the values in integer columns in ASCII tables, 
however many existing software packages only support values within the range 
of a signed 32-bit integer (-2147483648 to +2147483648).  This effectively 
limits the sizes of FITS images and tables (as given by the NAXISn keywords).

2. FITS does not officially support 64-bit twos-complement signed binary 
integers in either primary array images, image extensions, or columns in 
binary tables.

3. Informal proposals have been made to support 64-bit FITS images with 
BITPIX = 64 and 64-bit integer columns in binary tables with TFORMn = 'rK' 
(where 'r' here represents an integer repeat value).   It has also been 
proposed to support a 64-bit variable length array descriptor format in 
binary tables using a TFORMn keyword with data type = 'Q' (analogous to the 
existing 32-bit 'P' variable length array descriptor format).

4. Some FITS software packages support these informal proposals on a trial 
basis, however FITS file designers are urged to use these formats with 
caution.  These formats should only be used in cases where a) there is no 
reasonable alternative way to represent the information using standard FITS 
formats, and b) where it is reasonably certain that the software packages 
that are required to read the FITS files have been specifically designed to 
support the long integer FITS formats.

5.  It is possible that the FITS Standard might be modified in the future to 
officially support images with BITPIX = 64 and/or binary table columns with 
TFORMn = 'K' or 'Q'.  Developers of new FITS software may wish to anticipate 
this possibility and add compatibility with these possible future formats.


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