[AstroPy] Parameter 'origin' in WCS-methods

Michael Droettboom mdroe at stsci.edu
Fri May 8 08:48:34 EDT 2009

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).


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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