Re: UCD time.obs

Hi Silvia, Steve, Dick, Rob,

let me extract from the discussion on the UCD time.obs three points:

We should not mix up a "duration" (interval of time) with an "instant" (currently date/time), or different instants during the "duration". This semantic difference is reflected in the present list of UCDs.

1. time-instant: Astronomer are used to characterize their observations by a time-stamp (a UNIQUE time-stamp) that is reasonably to consider some sort of average between the beginning and the end of an observation. This stamp/label is called EPOCH of the observation. The corresponding UCD is time.epoch .
2. time-duration: First of all we need to consider that modern observations are often (I dare say always in the case of observations from space) the result of multiple exposures. While 30 years ago there was no semantic difference between observation and exposure, today we need to be precise, one observation being the result of more than one exposure, leading to an observation-duration usually greater than a single exposure-duration The UCDs time.obs and time.expo reflect this difference.
3. start/end, and the correspondance between FITS keywords and UCDs. I'm aware of the importance that FITS keywords still have in describing our data, but I'm also aware of the fact that (expecially in the "time" domain!!) there is a tendency to force (to forge?) the initial meaning and to proliferate keywords (Rob is right!). I think that we already have the UCDs corresponding to the quoted keywords: DATE-OBS = MJD-OBS : time.obs.start (time-stamp of the beginning of the observation) DATE-AVG = MJD-AVG : time.epoch[;obs] (epoch of the observation)

I also agree with Steve sentence

/> It would be dismaying if the UCDs/
/> and the FITS keywords became dissonant rather than consonant/
and Rob's comment to it,

but I interpret his warning as: we should not make the same mistakes!

Why start/end and not min/max: First of all for esthetic reasons. Most seriously, because in my opinion min/max whould indicate a min/max DURATION, while start/end are certainly referring, correctly, to two INSTANTs of time. Finally, start/end are the words that you find in the description of the data, not min/max.

Andrea

Andrea Preite Martinez                 andrea.preitemartinez-at-iasf-roma.inaf.it
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