[fitsbits] the need for BITPIX=64?

[Thierry Forveille]

William Pence writes:
 > 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
 > 
All of these examples are to some extent a bit academic: 
- histogram arrays and accumulated sums presumably have enough Poisson noise
  that no significance is lost by storing them as floats (64 bits floats
  if needed)
- tables look like a better match than images for arrays of time values.
- other sciences presumably have similar limitations to their dynamical
  ranges, or at least we haven't yet really heard of one that does not.
What's lacking for now is somebody that says "Hey, I (will) have a data set 
that I want to write to disk that requires 64 bit integers".


 > 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.
 > 
Part of the disagreement here is probably between the part of the community
that uses FITS for "everything" and would thus like it to cover every corner
case, and the part that sees it as an exchange format that they need to
keep a converter from and that they would thus like to keep reasonably simple.