[AstroPy] Bug in wcs_world2pix?

Michael Droettboom mdroe at stsci.edu
Fri Sep 19 10:17:03 EDT 2014

On 09/19/2014 10:09 AM, David Berry wrote:
> On 19 September 2014 15:01, Michael Droettboom <mdroe at stsci.edu> wrote:
>> On 09/19/2014 09:13 AM, David Berry wrote:
>>> On 19 September 2014 13:23, Michael Droettboom <mdroe at stsci.edu> wrote:
>>>> I've brought this to Mark Calabretta's attention -- he may have an idea.
>>> wcslib provides an option to switch this sort of range checking on or
>>> off. With pyast, I take the attitude that the answer to the question
>>> (in this case)  "what pixel has coords RA=211 Dec=-26" is "no pixel
>>> has those coords", and so pyast will always return "NaN" values in
>>> such circumstances. To return apparently usable values which are in
>>> fact wrong is just asking for problems in my opinion.
>> Yes, but the WCS range checking only happens for a TAN projection in the
>> pixel-to-sphere direction.  Not sure why that is.  In this particular
>> case, it's "garbage in", but "reasonable values out" from s2p, and then
>> p2s can't know to complain about it.  That might be something to improve
>> there.
> The docs for the wcsbchk function within wcslib says that if the
> "bounds" value is odd, then strict bounds checking is performed for
> the spherical-to-Cartesian (s2x) transformation for a TAN projection.
So it does.  Not enough coffee today.  Sorry about that.


Michael Droettboom
Science Software Branch
Space Telescope Science Institute


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