[fitsbits] DATE-OBS: circulate the WCS papers!

[Arnold Rots]

Within the High Energy community there is a fairly uniform usage of
DATE-OBS; it is the start of the observation. while the end is given
by DATE-END.  One can infer this definition by the presence of both
keywords, or more formally by the reference in the HDUDOC keyword.

  - Arnold

Doug Mink wrote:
> 
> Steve Allen wrote:
> > 
> > On Tue 2002-01-22T14:32:12 -0800, Steve Allen hath writ:
> > > In effect, we deem DATE-OBS to be broken/deprecated and aren't
> > > spending any engineering time fixing how we handle it.
> 
> > As a result we feel that we cannot re-engineer newer systems to assign
> > a precise meaning to DATE-OBS.  If we did it could be mistaken to
> > imply that the precise meaning applies to older instrument systems.
> > We feel that we must use privately-defined observation timing keywords
> > until such time as a WCS paper V or VI comes along to deal with
> > "Representations of temporal coordinates in FITS".
> 
> As someone who writes a widely-distributed analysis package, RVSAO, which
> utilizes both time and pointing direction keywords, in computing radial
> velocities from spectra, I hope that temporal coordinate agreements come
> sooner rather than later.  For my work, I need the midtime of the observation,
> which is usually computable from start or end times and exposure.  Sometimes
> an observation may have temporal gaps in it, making such a computed midtime
> wrong.  Temporal coordinates quickly get complicated enough to deserve as
> much of a paper-length treatment as spectral coordinates are getting.
> 
> I got so frustrated at the myriad combinations of keywords for timing,
> observing location, and pointing direction that I wrote an IRAF task to
> compute the velocity correction I needed accepting input in as many ways
> as I could think of.  Here is a web page describing what it looks for:
> 
>  http://tdc-www.harvard.edu/iraf/rvsao/bcvcorr/bcvcorr.proc.html
> 
> I also wrote a date conversion program as part of the WCSTools package
> which is described at
> 
>  http://tdc-www.harvard.edu/software/wcstools/getdate.html
> 
> -Doug Mink
>  Telescope Data Center
>  Smithsonian Astrophysical Observatory
>  Cambridge, Massachusetts
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> 

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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 81                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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