Question about a fits header

[Rob Seaman]

Bill Wyatt (wyatt at cfahub.harvard.edu) writes:

> Since there are 86,400 seconds in a day, and there was a DATE keyword,
> 52917 is surely is the second of the indicated day.

But is it UT or local time?  This corresponds to 14:41:57 or ~ 2:42 PM
if it's local time.  One can probably safely assume standard rather than
daylight savings time.

Note, however, that "52917" can also be interpreted as a legal ISO 8601
time string, representing ~ 5:30 AM, local time (no "Z", so not UT).
I don't know what ISO 8601 says about daylight time.

At least, as best I can tell it's legal to omit the leading zero for
the hours field.  The only constraint I see in the web summaries of the
standard is that the time cannot be truncated from the left - but only
when the combined date/time format is used, otherwise hyphens are used
as place keepers for missing fields.  The "T" is optional if the usage
is clear from the context (the name of the keyword is "OBS_TIME").

Does anybody have an official copy of the ISO 8601 standard that they
can bring to the ADASS meeting?  We may want to consult it before we adopt
it into FITS...  Hopefully the standard itself is more clear about issues
like omitting leading zeroes than the summaries are.

One concern I have about the ISO standards is that it is rather hard to 
find a copy without buying one yourself.  For instance, the University
of Arizona library doesn't appear to have a copy.  There may not be a
copy anywhere in Tucson - or Arizona, for that matter.  On the other
hand, the FITS standard has been propagated around the world wherever
there are copies of the A&A Supplements - or you can just get the
documents off the web.

The answer to the original question, by the way, is that only the authors
of this particular FITS file (or of the software that produced it) will
be able to reliably define what "OBS_TIME" means, since this is not a
standard FITS keyword.  If the authors can't be consulted, then reality
checks such as that optical/near IR data (not calibrations) must have
been acquired during local nighttime are about all you have to go on.
An hour angle or airmass keyword can be converted back into a sidereal
time if you have the telescope coordinates (celestial and geographic).

Rob Seaman
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seaman at noao.edu, http://iraf.noao.edu/~seaman
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