[AstroPy] pywcs issue

Yogesh Wadadekar yogesh@ncra.tifr.res...
Fri Mar 25 12:03:24 CDT 2011

> The WCS is not guaranteed to be defined outside of the image. 
> Particularly with SIN projections, it is often the case that the results 
> are invalid when outside of the image.  wcslib (and pywcs) does not do 
> any sanity checks for this.

Thank you for this clarification. One expects that the conversions will 
become progressively more inaccurate as one moves further out from the 
image. But it is strange that at the greatest possible distance away on 
the sphere (~180 degrees away), results are similar to what one sees at 
very small distances from the image center. I will check the SIN 
projection equations to get a feel for this; I have not seen this happen 
the more commonly used TAN projection.

We will, of course, use our own sanity checks to operationally overcome 
the problem.



Yogesh Wadadekar			 Tel: 91 20 25719238
National Centre for Radio Astrophysics   e-mail: yogesh@ncra.tifr.res.in
Post Bag 3, Ganeshkhind
Pune 411007, India

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