On Mar 2, 2006, at 4:13 PM, Steve Allen wrote:
> It would be dismaying if the UCDs and the FITS keywords became
> dissonant rather than consonant.
Most certainly agree! Wouldn't mind, however, if the FITS standards process were to be informed by VO standards, not just the other way around :-)
> DATE-OBS
> The current FITS standard asserts that this should be assume to refer
> to the start of an observation unless another interpretation is
> clearly explained in the comment field.
Have also seen headers with all variations of beginning, middle, and ending timestamps (and not all provided via DATE-OBS) and even both beginning and ending time stamps with no explicit exposure duration per se. Not clear that one size can, or should, fit all.
Should also point out that the syntax and semantics of DATE-OBS changed significantly due to Y2K. The original usage was to specify only a calendar date. Current usage can combine date and time into an atomic whole.
> DATE-AVG seems to correspond best to "time.obs;stat.mean".
Just as there are various statistical measures of central tendency, an "average" representative timestamp for an observation may reasonably mean many different things. Various instruments, for instance, allow pausing and resuming an exposure for arbitrary periods. The camera shutter (if any) may not even be open during the claimed midpoint of an observation. Other instruments, for instance drift scans, have significantly different representative timestamps for different pixels. A shuttered camera on a space platform may have one (scalar) timestamp for captured "science" photons and another (vector) timestamp for radiation events that continue to accumulate during readout. Even simple (ground-based, shuttered, etc.) observations might benefit from correcting the overt midpoint timestamp for varying airmass during the observation. (Alternately, an "effective" airmass might be computed.)
Hesitate to refer folks, yet again, to STC.
Rob Seaman
NOAO
Received on 2006-03-03Z00:55:35