Subject: Re: DATE-OBS='31/12/99'

From: seaman at noao.edu (Rob Seaman)
Date: 26 Jun 1996 20:29:42 GMT

Steve Allen writes:

> Take note, however, of the WCS draft paper [...] which proposes to
> standardize the usage of MJD-OBS.  This will work fine if the proposal
> also solves the backwards compatibility problem of existing FITS
> headers using different timescales.

Well, I'm not sure how the proposal could possibly do this.  Each
telescope and software package has traditionally had slightly (or
grossly) different ideas about time.  Hopefully the individual data
providers can supply conversion tools to whatever standard may be
established, but a lot of archival data was not acquired using an
accurate (or accurately defined) clock in the first place.

> A date format which does not also include the time of the observation
> increases the risk of "midnight" coding errors where the DATE and TIME
> are sampled on different "days".

I agree with the spirit of this statement, but an unsegmented keyword
doesn't remove the risk of these types of errors, since the keyword
still has to be formatted from the system clock(s).  On the other hand,
having dual DATE-OBS/UT keywords (or whatever) isn't necessarily risky.
Unix and other systems combine an unsegmented system time with builtin
formatting routines.

Ideally one would determine the unsegmented keyword (e.g., MJD-OBS) and
derive the DATE-OBS and UT from that.  MJD is not a very human friendly
format, and many types of pilot processing errors are likely if readable
calendar dates and clock times aren't immediately accessible to the
scientist.

> DATE-ISO does not distinguish between FITS 'DATE' and 'DATE-OBS'; i.e.,
> between the date of the construction of the FITS header and the date
> when the data were acquired.  However, it is a much more human-readable
> form than MJD.

By just allowing DATE as well as DATE-OBS (any others in wide usage?)
to use 4 digit years in addition to the current 2 digit years, this
problem is avoided.  I suppose the HDU creation date could also be
represented as MJD-HDU...

Rob Seaman

-- 
seaman at noao.edu, http://iraf.noao.edu/~seaman
NOAO, 950 N Cherry Ave, Tucson AZ 85719, 520-318-8248
PGP: 98 8D 8B 49 74 9A 41 88  3A 43 87 54 51 BF 30 4B