[AstroPy] surface computing on the sphere for projected maps

[Johann Cohen-Tanugi]

Hello, I have a 2D skymap resulting from a given projection (say 'CAR' 
for instance, but not necessarily), and I would like to compute the 
corresponding surface on the sphere of a pixel.
What is the best way to do that, with pywcs if possible, but any 
suggestion of other tools welcome.
thanks in advance,
Johann