Rob Seaman wrote:
Speaking of which, it is the duplicate keyword requirement that seems
most onerous. To implement this efficiently for all keywords, one
would have to build a hash table or some such for each header. Then
one is left with the question of what to do upon detecting a
duplicate. The sense of a requirement is to simply throw an error
and exit. How helpful is that?

This is missing the main point of this new requirement. No current
software system that I am aware of (except for the FITS verifier code)
checks for duplicated keywords, so users have no idea which of the
duplicated keywords is being used by a particular program. The software
might be using the first, the 'next', or the last instance of the
keyword. This could easily cause the user to derive incorrect
scientific results. What is the best way to prevent this from
happening? Seems to me we should focus on the root of the problem and
(formally at least) disallow duplicated keywords in a conforming FITS
file. This doesn't mean software should automatically throw out a file
that inadvertently has a duplicated keyword.

Stepping back a little, I think the seriousness of this problem depends
on what keyword is duplicated. If it is just some observatory-specific
keyword that does not directly affect the scientific results, then it
does not matter very much, and data providers need not worry about it.
But if a critical WCS keyword, or exposure time keyword is duplicated in
the file with different values, then surely the data providers need to
take responsibility and fix the problem.

Bill Pence
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__________________________________________________ __________________
Dr. William Pence
NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice)
Greenbelt MD 20771 +1-301-286-1684 (fax)