Std. date/time/RA/Dec FITS keywords?

[Peter Bunclark]

Steve Allen wrote:
> 
> In article <4mt60p$1pn at pecos.msfc.nasa.gov>,
> Peter Bunclark  <psb at ast.cam.ac.uk> wrote:
> >Tim DeBenedictis wrote:
> >> of different FITS-format images.  It would be nice to use the R.A., Dec.,
> >> date, and time information stored in the images' FITS headers, but
> >> unfortunately there does not seem to be any standard FITS keywords which
> >> provide this information...[deletia - mod.]
> 
> >Read :
> ><A HREF="http://fits.cv.nrao.edu/documents/documents.html" >FITS
> >Documents</A>
> 
> This reply may give the impression that there is a definitive answer
> to the question.  There is not.
> 
> There are no adopted standards specifying the keyword name or format
> of RA and Dec in a FITS header.  There exists decimal degrees, decimal
> hours, and hexadecimal strings in hours and degrees.  The given RA and
> Dec may be expressed in a catalog reference frame or as notably
> inaccurate telescope encoder readings.
> 
> Similarly there is no adopted standard specifying the keyword name or
> format of observation time in a FITS header.  Again, existing practice
> varies rather widely as do the accuracies of the clocks used to obtain
> the time.
> 
> There is a draft document at
> http://fits.cv.nrao.edu/documents/wcs/wcs.html
> whose adoption should solve this problem, but its recommendations
> are not generally found in any existing FITS headers.
> 
> --
> Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
> sla at ucolick.org      Voice: +1 408 459 3046     FAX: +1 408 454 9863
> WWW: http://www.ucolick.org/~sla                PGP public keys:  see WWW

I agree that there hasn't been much standardisation up to present; but
anyone writing new software should study the available documents on
wcs, including the section in NASA's ``A Users's Guide for the Flexible
Image Transport System (FITS)'',  and apply the recomendations to his
instrument/system.
	The disagreements seem to be in how exactly to represent the
image skew, rotation and scale.  The actual convention for representing
the coordinates of the reference pixel are as laid down in the
mystical PAPER I , ie CRVALn in degrees for ra and dec.  Meanwhile,
most observatories have actually used RA and DEC keywords in more or 
less human-readable format, but these shouldn't in fact be part of
a wcs.
	Since widely-distributed software (eg SAOTNG) is becoming available
to exploit images with wcs, there is much more motivation than
previously to exploit the potential.

Peter.