[AstroPy] Determining when world and pixel axes are independent

[Perry Greenfield]

On Jan 13, 2014, at 2:11 PM, Thomas Robitaille wrote:

> I have the same question. Just brainstorming here since I don't know
> the final answer, but I guess there are (as you said) two things going
> on here - one being the fact that certain projections can couple
> coordinates (such as RA/Dec) in some projections (e.g. TAN), and the
> other being rotations of coordinate systems relative to the pixel
> axes. For the latter, I *guess* one could look at the non-diagonal
> elements of the PC matrix:
> 
>>>> w = WCS(...)
>>>> w.wcs.get_pc()
>    array([[ 1.,  0.,  0.],
>           [ 0.,  1.,  0.],
>           [ 0.,  0.,  1.]])
> 
> in the above case, there is no rotation. But for axes 1 and 2 the
> projection is not -CAR:
> 
>    In [9]: w.wcs.ctype
>    Out[9]: ['RA---SFL', 'DEC--SFL', 'VOPT']
> 
> so axes 1 and 2 are linked (but axis 3 is independent). Maybe the
> solution to find if an axes is independent of the others is:
> 
> - check that the non-diagonal elements for that axis are zero
> - check that the projection for that axis is not a spherical
> projection, unless it's -CAR in which case it's also fine
> 
> What do others think? Is this robust? Or are there projections that
> are not the usual spherical projections that might link axes? Does one
> need to also check that there are no distortion coefficients in
> addition to the above criteria?

No, it is not robust. Phil basically explains why. If the WCS involves expressions that use more than one axis, there presumably is coupling between more than one axis. Many of the intrinsic projections have intrinsic coupling (e.g. TAN).


Perry