[fitswcs] Status of WCS negotiations

[Don Wells]

Mark Calabretta writes:
 > Subject: PLON/PLAT
 > 
 > Recent discussion on sci.astro.fits has indicated that it would be useful to
 > formally recognize a PLON/PLAT coordinate system in WCS..
 > Note that PLON/PLAT is simply an application of the methods already
 > defined in the WCS draft using the xLON/xLAT provision.  The
 > proposal here is effectively to register the name of this
 > application; it does not constitute a fundamental change to WCS
 > (for example, it won't require a change to WCSLIB).. 
 > PLON/PLAT would have the following defining characteristics:
 >    1) Used mainly in conjunction with the azimuthal perspective projection,
 >       AZP (with mu << -1), for planetary/solar mapping.
 >    2a) The PC matrix is chosen so that the perimeter of the sphere has unit
 >       radius..
 >    2c) The CDELTn header cards scale (x',y') to (x,y) in the "degree" units
 >       required by WCS.  Their signs are set to invert the native longitude as
 >       appropriate for a sphere observed from the outside as opposed to the
 >       usual case of the celestial sphere observed from the inside..

Is there any reason why this transformation cannot be represented by
CDij?  I agree that the product of the CDELTn vector and PC matrix,
which are each understandable entities, will be a matrix whose content
will be inscrutable, but it will be written and read by software, not
humans, and anyway the decomposition of the matrix can be put into
comments in the header.

-Don