[AstroPy] Parameter 'origin' in WCS-methods

Erin Sheldon erin.sheldon at gmail.com
Fri May 8 09:12:27 EDT 2009

On Fri, May 8, 2009 at 8:48 AM, Michael Droettboom <mdroe at stsci.edu> wrote:
> The distortion table lookup is non-invertible (without doing some sort
> of iterative approximation) -- so that's unimplemented.  There are some
> good examples in Paper IV as to why it isn't really possible in a
> general sense -- but it may be reasonable assuming that the lookup table
> fits certain characteristics.
> SIP is only invertible if a set of inverted coefficients is provided in
> the header, which we support but don't require.
> So it's more "difficult" than "impossible" -- it just isn't something
> we've had an immediate need for here (which is primarily within the new
> version of MultiDrizzle).

I've got some wcs code that demonstrates how to do this:


The sky2image method of the WCS class is relevant here.  By default it
uses scipy.optimize.fsolve to get the inverse.

Erin Sheldon
Brookhaven National Laboratory

> Mike
> Martin Kuemmel wrote:
>> Thanks for explaining the 'origin'. Actually I had read
>> these pages, but not carefully enough, obviously
>> Next question:
>> There is the method 'all_pix2sky(origin, *args)' to get sky coordinates,
>> which includes "everything" (SIP, distortion, wcslib). There is no
>> method 'all_sky2pix(origin, *args)'.
>> Is it planned to have to implement this? Or is it difficult/impossible?
>> Cheers,
>> Martin
> --
> Michael Droettboom
> Science Software Branch
> Operations and Engineering Division
> Space Telescope Science Institute
> Operated by AURA for NASA
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