[fitsbits] polar coordinates

[David Berry]

On 11 December 2013 19:13, William Thompson <William.T.Thompson at nasa.gov> wrote:
> We have a similar situation in solar astronomy.  One commonly used
> coordinate system expresses coordinates in elongation angle from Sun
> center, and the position angle counter-clockwise from the solar north
> pole.  To work this into the WCS, I had to map the elongation angle E
> into a latitude defined as E-90, i.e. make Sun-center the south pole of
> the system.  I chose the south pole instead of the north pole, so that
> the angle still increased in the usual fashion.  It's not an optimal
> solution, but it does make it WCS-compatible.

How do you encode the shift to put latitude zero at the south pole?

> The part about taking the log of the polar angle should be fairly easy
> to incorporate into WCS.  This is just a new spherical projection, which
> would have to be properly defined and signaled with a unique projection
> code.  There is a precedent to adding new projections which are
> otherwise compatible with the WCS formalism.

"Easy" is not a word often associated with trying to get changes to
the FITS-WCS standard agreed and implemented! And I'm not sure having
a separate projection code for every possibly interesting variant is a
good idea.

David