# [AstroPy] Bug in wcs_world2pix?

David Berry d.berry at jach.hawaii.edu
Fri Sep 19 08:14:33 EDT 2014

```Am I missing something? This is a TAN projection centred on CRVAL1=16,
CRVAL2=23. Since it is a TAN projection, the projection plane only
includes half the sky - i.e. the hemisphere centred on (16,23). But
the test point (211,-26) is not contained within this hemisphere and
so is not included in the TAN projection plane. In other words, no
pixel has the coordinates RA=211 Dec=-26, and so the results of  using
wcs.wcs_world2pix should be NaN, or "invalid", or something.

Unless I'm missing something...

David

On 19 September 2014 12:58, Bob Garwood <bgarwood at nrao.edu> wrote:
> I suspect what's going on here has to do with the specific CD matrix, which
> includes rotation. I think because of that rotation the world coordinate
> actually maps back in to the grid at the antipode. I haven't checked this by
> hand to see if that's the case.
>
> Bob
>
> On September 19, 2014 7:47:30 AM EDT, Howard Bushouse <bushouse at stsci.edu>
> wrote:
>>
>> Huh. But that’s not how astronomers measure RA and Dec, even when
>> translated to degrees. If I’ve got an object with an RA of 17:45:00.0
>> (hh:mm:ss.w) the RA in degrees is 266.25. We *never* use a negative RA.
>> Instead of a range of -180 to 180, we use 0 to 360. Dec is the only one that
>> ever goes negative (-90 to +90, as you indicated).
>>
>> -hb
>>
>> On Sep 18, 2014, at 8:26 PM, Michael Droettboom <mdroe at stsci.edu> wrote:
>>
>>>  The range of ra and dec is (-180, 180) and (-90, 90) respectively.
>>>  (211, -26) is equivalent to (31, 26).
>>>
>>>  Mike
>>>
>>>  On 09/18/2014 05:04 PM, Maik Riechert wrote:
>>>>
>>>>  Hi,
>>>>
>>>>  I'm having some great trouble currently and am a bit confused to!
>>>>  o:
>>>>
>>>>
>>>>  x,y = wcs.wcs_world2pix(211, -26, 0)
>>>>  print x,y # 836.316942718 658.26364248
>>>>  ra,dec = wcs.wcs_pix2world(x, y, 0)
>>>>  print ra, dec # 31.0 26.0
>>>>
>>>>  It seems like world2pix gives me bogus results. I was expecting values
>>>>  way outside my image area (image size is 4256x2832). Am I doing
>>>>  something obvious wrong?
>>>>
>>>>  The WCS headers can be seen at http://pastebin.com/JrdiL949
>>>>
>>>>  Thanks for the help,
>>>>  Maik
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>>>
>>>
>>>
>>>  --
>>>  Michael Droettboom
>>>  Science Software Branch
>>>  Space Telescope Science Institute
>>>
>>>  http://www.droettboom.com
>>>
>>> ________________________________
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>>>  AstroPy at scipy.or!
>>>  g
>>>
>>> http://mail.scipy.org/mailman/listinfo/astropy
>>
>>
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```