[fitsbits] Az/El and moving body basis system support

[Steve Allen]

On Thu 2002-04-18T15:44:56 +0200, Jennifer Hatchell hath writ:
> In WCS FITS, the first 4 characters of CTYPEj give the basis coordinate
> system, but as far as I can see the choices are limited to RA/DEC,
> GLON/GLAT, ELON/ELAT, HLON/HLAT or SLON/SLAT.  Can anyone advise on how
> to deal with the following:
>
> - output in azimuth and elevation
>
> - tracking a solar system body, where the body's coordinate frame
> rotates with respect to the standard astronomical frames

The drafts of WCS paper II also allow for other undefined spherical
systems using CTYPE cards in pairs of the form xLAT, xLON, and xyLT,
xyLN.  They also point out that alt-az is just another form of LAT/LON
with an odd sign convention.

There are existing examples of several such systems by various
authors.  Calabretta suggests SELT, SELN for lunar (selenographic)
coordinates.  Calabretta and I have suggested TLAT, TLON for
terrestrial coordinates.  Thompson has suggested CRLN, HGLN, HPLN, and
HRLN as various of the longitude forms for use in solar astronomy.

None of these is standardized, and it is not likely that the WCS
papers will attempt to do so.  On the other hand, there is no urgent
need to standardize, for the handling of any such coordinates is
explicit in the standard.  The exact meaning of the CTYPE abbreviation
can be provided in the WCSNAME keyword.

--
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.org      Voice: +1 831 459 3046     http://www.ucolick.org/~sla
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