[AstroPy] Problem running pywcs example

Michael Droettboom mdroe@stsci....
Fri Aug 20 14:33:19 CDT 2010


Indeed, the problem you describe exists, because sub() merely swaps all 
of the parameters for each of the axes.

I have a proposed solution in SVN which adds a new kwarg "ra_dec_order" 
to all the methods that convert to/from sky coordinates.  When 
"ra_dec_order" is True, the input and output sky coordinate vectors will 
always be of the form (ra, dec) regardless of the order of the CTYPE 
keywords.

I'd consider this functionality totally experimental until we have a 
chance to play with it and see if it's a good idea or not.

Are you able to build pywcs yourself from SVN?  It would be great to 
have your feedback.  The svn address is:

http://svn6.assembla.com/svn/astrolib/trunk/pywcs

Contact me off list if you need instructions building/installing it.

Mike

On 08/20/2010 10:08 AM, Peter Erwin wrote:
> Hi Mike,
>
> You should be able to download it directly from the SDSS archive, via:
>
> http://das.sdss.org/imaging/94/40/corr/2/fpC-000094-g2-0220.fit.gz
>
>
> cheers,
>
> Peter
>
> On Aug 20, 2010, at 3:42 PM, Michael Droettboom wrote:
>
>    
>> Peter,
>>
>> Can you send me one of your FITS files with CTYPE1 == "DEC--TAN" so I
>> can investigate this further?
>>
>> Mike
>>
>> On 08/20/2010 08:42 AM, Peter Erwin wrote:
>>      
>>> Hi Mike,
>>>
>>> Thanks for the "sub" suggestion... however, that seems to have the side effect of reversing
>>> the x,y output order from new_wcs.wcs_sky2pix when the FITS header has CTYPE1="DEC--TAN"
>>>
>>> i.e., the output from new_wsc.wcs_sky2pix(ra, dec) is [ array([y]), array([x]) ] when
>>> CTYPE1="DEC--TAN", instead of [ array([x]), array([y]) ]
>>>
>>> cheers,
>>>
>>> Peter
>>>
>>> On Aug 19, 2010, at 8:24 PM, Michael Droettboom wrote:
>>>
>>>
>>>        
>>>> I've added a note in the documentation about this.
>>>>
>>>> Also of note is the "sub" method which creates a new WCS with the axes
>>>> in the order you specify.  Therefore, given an arbitrary WCS where you
>>>> don't know the order of the axes (or even how many axes it has), you can do:
>>>>
>>>>     new_wcs = wcs.sub(['latitude', 'longitude'])
>>>>
>>>> to get a new wcs that will always work in (ra, dec).
>>>>
>>>> http://stsdas.stsci.edu/astrolib/pywcs/api_wcs.html#pywcs.WCS.sub
>>>>
>>>> Mike
>>>>
>>>> On 08/19/2010 12:22 PM, Peter Erwin wrote:
>>>>
>>>>          
>>>>> Hi Mike,
>>>>>
>>>>> It probably would be useful to mention the units explicitly in the documentation, yes.
>>>>>
>>>>> Your note about the RA,Dec order depending on the CTYPE arguments does help
>>>>> explain some weird, inconsistent results I was getting for SDSS images (some of which
>>>>> turned out to have CTYPE1="DEC--TAN", while the majority have CTYPE1="RA--TAN").
>>>>> That explains why WCS.wcs_pix2sky was returning (RA, Dec) for some images and
>>>>> (Dec, RA) for others.
>>>>>
>>>>> It might be useful putting a short note about this in the documentation, since I expect
>>>>> a fair number of people will be like me (i.e., expecting things to be RA,Dec by default
>>>>> and unaware that this can be mixed up depending on the details of the image header).
>>>>>
>>>>> cheers,
>>>>>
>>>>> Peter
>>>>>
>>>>> On Aug 19, 2010, at 4:11 PM, Michael Droettboom wrote:
>>>>>
>>>>>
>>>>>
>>>>>            
>>>>>> On 08/19/2010 09:23 AM, Michael Droettboom wrote:
>>>>>>
>>>>>>
>>>>>>              
>>>>>>> On 08/18/2010 03:56 PM, Peter Erwin wrote:
>>>>>>>
>>>>>>>
>>>>>>>                
>>>>>>>> Hi Mike,
>>>>>>>>
>>>>>>>> Thanks for the quick reply!
>>>>>>>>
>>>>>>>> It looks like what happened is that *was* an earlier version of pywcs hiding
>>>>>>>> in /Users/erwin/lib/python [as opposed to
>>>>>>>> /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6 ,
>>>>>>>> which is where pywcs 1.9 got installed].  Possibly it got installed as part of
>>>>>>>> an earlier installation of stsci_python ...
>>>>>>>>
>>>>>>>> Anyway, I nuked the earlier installation, and now pywcs seems to work.
>>>>>>>>
>>>>>>>>
>>>>>>>> Am I missing something, or does the documentation for pywcs mention anywhere what
>>>>>>>> "sky coordinates" actually means (i.e., allowed formats, types, order, etc.?).
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>                  
>>>>>>> The order is described in the wcs_sky2pix docstring:
>>>>>>> """
>>>>>>> Either two or three arguments may be provided.
>>>>>>> 	• 2 arguments: An N x naxis array of x- and y-coordinates, and an origin.
>>>>>>> 	• 3 arguments: 2 one-dimensional arrays of x and y coordinates, and an origin.
>>>>>>> """
>>>>>>>
>>>>>>> As for types, it may be any numeric type (and the underlying calculations are performed in doubles).
>>>>>>>
>>>>>>>
>>>>>>>                
>>>>>> And for additional clarification, I should add that the order (whether it's [dec, ra] or [ra, dec]) depends on the CTYPEia arguments in the FITS header.  You can determine that programmatically (if necessary) from the .lat, .lng, .lattype and .lngtype members.
>>>>>>
>>>>>> Mike
>>>>>>
>>>>>> -- 
>>>>>> Michael Droettboom
>>>>>> Science Software Branch
>>>>>> Space Telescope Science Institute
>>>>>> Baltimore, Maryland, USA
>>>>>>
>>>>>> _______________________________________________
>>>>>> AstroPy mailing list
>>>>>> AstroPy@scipy.org
>>>>>> http://mail.scipy.org/mailman/listinfo/astropy
>>>>>>
>>>>>>
>>>>>>              
>>>>> =============================================================
>>>>> Peter Erwin                   Max-Planck-Insitute for Extraterrestrial
>>>>> erwin@mpe.mpg.de              Physics, Giessenbachstrasse
>>>>> tel. +49 (0)89 30000 3695     85748 Garching, Germany
>>>>> fax  +49 (0)89 30000 3495     http://www.mpe.mpg.de/~erwin
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> AstroPy mailing list
>>>>> AstroPy@scipy.org
>>>>> http://mail.scipy.org/mailman/listinfo/astropy
>>>>>
>>>>>
>>>>>            
>>>> -- 
>>>> Michael Droettboom
>>>> Science Software Branch
>>>> Space Telescope Science Institute
>>>> Baltimore, Maryland, USA
>>>>
>>>> _______________________________________________
>>>> AstroPy mailing list
>>>> AstroPy@scipy.org
>>>> http://mail.scipy.org/mailman/listinfo/astropy
>>>>
>>>>          
>>> =============================================================
>>> Peter Erwin                   Max-Planck-Insitute for Extraterrestrial
>>> erwin@mpe.mpg.de              Physics, Giessenbachstrasse
>>> tel. +49 (0)89 30000 3695     85748 Garching, Germany
>>> fax  +49 (0)89 30000 3495     http://www.mpe.mpg.de/~erwin
>>>
>>>
>>>
>>> _______________________________________________
>>> AstroPy mailing list
>>> AstroPy@scipy.org
>>> http://mail.scipy.org/mailman/listinfo/astropy
>>>
>>>        
>>
>> -- 
>> Michael Droettboom
>> Science Software Branch
>> Space Telescope Science Institute
>> Baltimore, Maryland, USA
>>
>> _______________________________________________
>> AstroPy mailing list
>> AstroPy@scipy.org
>> http://mail.scipy.org/mailman/listinfo/astropy
>>      
> =============================================================
> Peter Erwin                   Max-Planck-Insitute for Extraterrestrial
> erwin@mpe.mpg.de              Physics, Giessenbachstrasse
> tel. +49 (0)89 30000 3695     85748 Garching, Germany
> fax  +49 (0)89 30000 3495     http://www.mpe.mpg.de/~erwin
>
>
>
> _______________________________________________
> AstroPy mailing list
> AstroPy@scipy.org
> http://mail.scipy.org/mailman/listinfo/astropy
>    


-- 
Michael Droettboom
Science Software Branch
Space Telescope Science Institute
Baltimore, Maryland, USA



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