[AstroPy] Changing dimensionality of WCS

Perry Greenfield stsci.perry at gmail.com
Tue Sep 24 16:14:44 EDT 2013

Keep in mind that something like this will only work if the coordinates being sliced are separable. For example, if the WCS involve a rotation of x,y, it doesn't make much sense to extract the wcs for x only since it requires as input both coordinates.

Given that, I'm not sure slicing is a good approach. This would need some careful thought as far as a user interface goes (sure, you can raise an exception when it doesn't work, but is there a simple way to see what coordinates are coupled?)


On Sep 24, 2013, at 4:06 PM, James Turner wrote:

> I don't know the answer re. astropy.wcs but it would be good if
> one could ultimately slice an nddata object and have the
> dimensionality and WCS reduced automatically if applicable (a bit
> like in IRAF but, hopefully, better) :-). It seems that an NDData
> can already be sliced directly but "support for WCS and units is
> not yet implemented". Maybe some more WCS manipulation routines
> are needed first...
> Cheers,
> James.
> On 24/09/13 15:49, David Berry wrote:
>> On 24 September 2013 19:23, Adam Ginsburg <adam.ginsburg at colorado.edu> wrote:
>>> When working with data cubes, I often need to extract WCS information
>>> for 1 or 2 of the WCS dimensions.   The docs
>>> (http://docs.astropy.org/en/latest/wcs/index.html) give decent
>>> examples of how to generate a WCS from scratch, but I don't see any
>>> obvious way to extract a lower-dimensional WCS from a WCS.  Does such
>>> functionality exist?  If not, does it make sense to generate a WCS
>>> from scratch and copy over just the relevant parameters?
>> This is an example of the general problem - given two arbitrary
>> coordinate systems, one with N axes and one with M axes, is there a
>> transformation between them, and if so, what is it? It is addressed in
>> pyast by the "convert" method. If you need ideas for an API see
>> http://starlink.jach.hawaii.edu/docs/sun211.htx/node245.html
>> Implementing this sort of feature in a general way is tricky but can
>> be done. For simpler cases where you just want to pick a specified
>> subset or superset of axes from a coordinate system, pyast uses
>> "pickaxes". See
>> http://starlink.jach.hawaii.edu/docs/sun211.htx/node348.html
>> I know pyast is not astropy, but I point these out in case ideas are
>> need for how to approach these issues.
>> David
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