[AstroPy] Bug in wcs_world2pix?

Maik Riechert maik.riechert at arcor.de
Fri Sep 19 10:57:14 EDT 2014


Just to add some context of what I'm doing. I got the star constellation 
data from Marshall Perrin the other day on this list. And now I'm trying 
to draw these constellations onto an image using WCS data. That means, 
for each constellation, I convert the RA,DEC points to pixel coordinates 
using wcs_world2pix, then I check whether all points are outside the 
image bounds (in that case I skip drawing it), if they are partly or all 
inside, then I draw the constellation.

So yes, I think the best way would be to return NaN in these cases. 
Otherwise I wouldn't know how to easily filter out those wrong points.

Cheers
Maik


On 19/09/14 15:14, Thomas Robitaille wrote:
> Ah of course, this is indeed what is going on here! I agree that it's
> worth thinking about whether cases like this should return an error.
> In the past (in WCSAxes) I've simply checked if the coordinate
> round-trips in order to determine if it's in the observational plane,
> but it may be more sensible to simply return NaN.
>
> Cheers,
> Tom
>
> On 19 September 2014 14:14, David Berry <d.berry at jach.hawaii.edu> wrote:
>> Am I missing something? This is a TAN projection centred on CRVAL1,
>> CRVAL2#. 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!1 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:
>>>>>>
>>>>>>
>>>>>>   wcs =CS(header)
>>>>>>   x,y =cs.wcs_world2pix(211, -26, 0)
>>>>>>   print x,y # 836.316942718 658.26364248
>>>>>>   ra,dec =cs.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
>>>>>> ________________________________
>>>>>>
>>>>>>   AstroPy mailing list
>>>>>>   AstroPy at scipy.org
>>>>>>   http://mail.scipy.org/mailman/listinfo/astropy
>>>>>
>>>>>
>>>>>   --
>>>>>   Michael Droettboom
>>>>>   Science Software Branch
>>>>>   Space Telescope Science Institute
>>>>>
>>>>>   http://www.droettboom.com
>>>>>
>>>>> ________________________________
>>>>>
>>>>>   AstroPy mailing list
>>>>>   AstroPy at scipy.or!
>>>>>   g
>>>>>
>>>>> http://mail.scipy.org/mailman/listinfo/astropy
>>>>
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