[fitsbits] the need for BITPIX=64?

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
NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice)
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