[AstroPy] Problem running pywcs example

Michael Droettboom mdroe@stsci....
Fri Aug 20 08:42:41 CDT 2010


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



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