[fitsbits] the need for BITPIX=64?

[William Pence]

Preben Grosbol wrote:
> I still have reservation concerning BITPIX=64 for the following reasons:
>   1) there seems no good physical reason for 64-bit integer images.  The
>       number of photons from astronomical source hardly justifies it
>       especially considering their statistical distribution.  Let someone
>       present a real, practical case and we should considere it.

Do you not consider any of the 15 cases given in my email of 07-June-2005 
real or practical?  This included:

   - histogram arrays derived from very large databases
   - arrays of measured time values
   - arrays of 'accumulated sums'
   - the need to import data from other sciences (space physics,
     planetary research, earth sciences) into FITS

I think it is important here to not set the requirements for justifying 
BITPIX = 64 arrays too high.  This data type will probably never be very 
widely used, but that is not the point.  All that should matter is that 
there is at least 1 case, important to some subset of the astronomical 
community, where having 64-bit integer arrays in FITS would be very useful. 
Also, the FITS format is used for many utilitarian purposes, so one should 
not automatically rule out more 'practical' uses, (e.g., temporary storage 
of 'scratch' arrays of intermediate computations) just because they are not 
based on fundamental scientific or physical needs.

>   2) The FITS standard is useful because the vast majority of systems
>        implements it - that is if one writes a conforming FITS file the
>        likelihood of reading it on any system is high.
>        Adding BITPIX=64 would require changes at the top level of all readers.
>        In order for this to actually be implemented people would have to feel
>        the need otherwise it remains empty words.

I think this overstates the negative impact of this change to FITS.  The 
vast majority of existing software will not need to be changed at all, since 
most software is designed to operate on specific data products from specific 
missions or instruments.  Very few, if any, operating missions will decide 
to change the format of their data products just because a new 64-bit 
integer format becomes available.  For example, the AIPS software package 
will probably not need to support 64-bit integer arrays, because radio 
observatories are not suddenly going to start producing data using that 
format.  A few new missions might decide to use the new 64-bit format in 
some cases, but they will also generally provide new analysis software that 
supports their data.

Bill Pence
-- 
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Dr. William Pence                          William.D.Pence at nasa.gov
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