[AstroPy] Determining when world and pixel axes are independent

[Perry Greenfield]

On Jan 13, 2014, at 2:56 PM, Phil Hodge wrote:

> Tom,
> 
> On 01/13/2014 02:46 PM, Thomas Robitaille wrote:
>> Just to clarify, I think we are agreeing here - I mentioned:
>> 
>> "- check that the projection for that axis is not a spherical
>> projection, unless it's -CAR in which case it's also fine"
>> 
>> In the example you and Phil gave, if the non-diagonal elements are
>> zero but the projection is TAN, this violates the rule above. My point
>> was that if any of the the diagonal elements are non-zero, or if the
>> projection is one of the projections of a sphere, i.e. any projections
>> from paper 2, (except CAR), then this means the axes are coupled, but
>> should one be ok if all non-diagonal elements are zero*and*  the
>> projection is CAR or a projection that is not one of the spherical
>> projections (but could be e.g. a spectral projection convention). Is
>> this correct?
> 
> Well, the Mercator projection should be OK, as spherical projections go, 
> and there are others.  But if there are distortions in addition to any 
> projection, that would make it pretty hard to determine whether the axes 
> were independent just by looking at the WCS attributes.
> 
> Phil

Right. But I think we all essentially agree. It's the details of the specific cases that takes all the work. For the most part, one could practically assume that spatial coordinate are coupled given the arbitrary orientation of the detectors and that we are mapping to the sky. The number of real uses of an aligned separable spatial system is so rare as to make it a waste of time to handle specially.

Perry