[AstroPy] Problem running pywcs example

Peter Erwin erwin@mpe.mpg...
Fri Aug 20 07:42:30 CDT 2010


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
>> 
>> 
>> 
>> _______________________________________________
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>> 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





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