[fitsbits] Proposed Changes to the FITS Standard

[Doug Tody]

I'll just second this.  I have seen many cases where a FITS header gets
multiple keyword instances after it has been modified by different
programs.  This is awful of course as the header is then ambiguous,
but it happens.  Software may or may not use the last instance as
a runtime value; for example, if the software does a linear search
through the header it is possible that it will take the first value
encountered (for either reading or writing) as this is more efficient,
and there are not supposed to be multiple keyword instances.  If the
software does this on a write, then it is the *first* value in the
header which will have the correct value.

We should strongly recommend that software which writes to a header not
create multiple values.  If there already *are* multiple values, frankly
it is not often clear what to do, as one does not like to delete
information from a header.

If we ever do a FITS 2.0 we should revisit this, but it is hard to
do more at this point for standard FITS than recommend that multiple
values not be allowed in the first place.  Once they are there you have
a problem.  - Doug



On Fri, 17 Aug 2007, Jonathan McDowell wrote:

> I agree with Thierry that there are many files which have repeated keywords,
> but I agree with another poster that there are existing implementations which
> assume it's the first instance, not the last instance, which prevails.
> So I think we should just strongly deprecate (not ban, and not impose
> an interpretation).
>  - Jonathan McDowell
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