[AstroPy] WCS solutions from table HDUs

[Simon Walker]

Hi,

When I try to read the wcs header from a table HDU, the returned wcs object is not valid (i.e. it maps pixel coordinates to pixel coordinates.)

My understanding is that `astropy.wcs` uses `wcslib` under the hood to handle most of the implementation, and `wcslib` supports table HDUs just fine. An example astrometric solution is given below:

TCTYP3  = 'RA---ZPN'           / WCS projection
TCTYP6  = 'DEC--ZPN'           / WCS projection
TCRVL3  =     307.241977403642 / RA at reference pixel
TCRVL6  =     25.3168420637255 / Dec at reference pixel
TCRPX3  =         1024.4417725
TCRPX6  =          1023.831605
TC3_3   = -0.00140653080291281
TC6_6   = -0.00140646456957538
TC3_6   =  1.3189450212842E-05
TC6_3   = -1.3038301062367E-05
TV6_1   =                   1.
TV6_3   =     9.26153828229481
TV6_5   =    -938.934425623138
TV6_7   =     688883.879115836

I’ve seen these keywords mentioned in the Paper I description, and I believe they are valid. The solution creates a valid astrometric solution when using the equivalent image wcs keywords (CD1_1, CRPIX etc.).

If this is not supported I’d be happy to attempt an implementation - I am relatively familiar with `wcslib`. My knowledge of the standard however is a little shaky.

Thanks,
Simon Walker