Re: Request for modification of UCD: time.obs

On Thu 2006-03-02T12:52:39 +0000, Silvia Dalla hath writ:
> I propose modifying 'time.obs'.
>
> This UCD is currently defined as: 'Observation on-time, duration'
>
> However we are missing a UCD that identifies the time stamp associated
> with an observation: for example if data are collected between a
> 'time.obs.start' and a 'time.obs.end', frequently the mid point of
> this time interval is used to define the time of the observation.
>
> I propose that 'time.obs' should be 'time stamp associated to the
> observation'.
>
> A new UCD could be used to identify the duration of the observation:
> the obvious choice would be 'time.obs.duration'.

There are several FITS reserved keywords with definitions which are closely related to these UCDs. It would be dismaying if the UCDs and the FITS keywords became dissonant rather than consonant. First a quick review...

>From the original Wells et al. paper there is
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. In the original paper by Wells et al. this keyword was defined simply as "date of data acquisition". As a result not all instances of its use conform to the suggestion (added nearly two decades afterwards) that it refer to the start of an observation.

FITS WCS Paper II (celestial coordinates) reserved MJD-OBS
with semantics identical to DATE-OBS except for being expressed as a floating point count of days rather than as a character string indicating a calendar date.

FITS WCS Paper III (spectral coordinates) reserved MJD-AVG
and
DATE-AVG
with semantics indicating that the value should refer to an average date (of a presumably extended observation).

Now I switch back to UCDs.

DATE-OBS seems to correspond best to Silvia Dalla's suggestion for a change to "time.obs".

DATE-AVG seems to correspond best to "time.obs;stat.mean".

But that calls up a question of whether the current semantics for "time.obs", which indicate a duration, are not begging for a new UCD entry in the stat group -- should there be a UCD something like "stat.range" or "stat.span" to indicate the difference between the maximum and minimum of a quantity?

I wonder why there is
"time.obs.start" instead of "time.obs;stat.min" and similarly
"time.obs.end" instead of "time.obs;stat.max"

But upon reading closely through
http://www.ivoa.net/Documents/latest/UCDlist.html I am left mystified by the distinction in semantics between the UCDs "time.obs" and "time.expo"
Is there a more detailed explanation of the difference in the semantics of these two quantities?

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