[AstroPy] Error in pixel to sky coordinate conversion with astropy.wcs

[David Berry]

That FITS file looks like it has a SCAMP header, that includes
distortion. SCAMP uses a basic TAN projection code but includes lots
of PVi_j keywords to specify the distortion terms. It uses the
distortion scheme described in an early draft of FITS-WCS paper2. AST
(and therefore pyast) support it, but I'm not sure if astropy.wcs
supports it.

David

On 29 April 2014 18:38, Tim Jenness <tim.jenness at gmail.com> wrote:
> I'm not sure what this telescope is intending but the DATE-OBS header is not
> at all compliant with the FITS standard so astropy is within its rights to
> complain. DATE-OBS must be in ISO date format if HH:MM::SS is being
> specified. Slashes are only used for pre Y2K format headers that just
> specified the day.
>
> I looked at the file with pyast and get the following warnings so it does
> manage to read it (and ignores the broken DATE-OBS so guesses the date:
>
> Reading your image produced the following messages:
>
> Result of attempt to read WCS encoded as: FITS-IRAF
>
> Succeeded
>
> ASTWARN = '        '
> ASTWARN = 'The original FITS header did not specify the longitude of '
> ASTWARN = 'the native north pole. A default value of 180 degrees was '
> ASTWARN = 'assumed.'
> ASTWARN = '        '
> ASTWARN = '        '
> ASTWARN = 'The original FITS header did not specify the date of '
> ASTWARN = 'observation. A default value of J2000 was assumed.'
> ASTWARN = '        '
>
>
>
> On Tue, Apr 29, 2014 at 10:20 AM, Angelo Varlotta
> <avarlotta at cfa.harvard.edu> wrote:
>>
>> Hello,
>> I need to convert a list of sources from pixel coordinates to sky
>> coordinates for some PAIRITel data, using astropy (0.3.1) and using the
>> following code:
>>
>> from __future__ import division
>> import numpy
>> from astropy import wcs
>> from astropy.io import fits
>> import sys
>>
>> hdulist = fits.open(sys.argv[-1])
>> w = wcs.WCS(hdulist[0].header)
>> pixcrd = numpy.array([[0,0],[24,38],[45,98]], numpy.float_)
>> world = w.wcs_pix2world(pixcrd, 1)
>> print world
>>
>> (the fits file is at h_long_CYGX3.1.11_coadd.fits, the code is in a file I
>> called pix2sky.py)
>>
>> The code derives from the first example in the documentation for
>> astropy.wcs at: http://docs.astropy.org/en/stable/wcs/index.html. While I
>> can do this sucessfully with a fits file generated from Fermi analysis (so I
>> know that astropy is installed properly and works), with the PAIRITel data I
>> get the following error message, after running from the terminal prompt
>> python pix2sky.py h_long_CYGX3.1.11_coadd.fits:
>>
>>
>> WARNING: FITSFixedWarning: RADECSYS= 'ICRS ' / Astrometric system
>> RADECSYS is non-standard, use RADESYSa. [astropy.wcs.wcs]
>> WARNING: FITSFixedWarning: 'datfix' made the change 'Invalid parameter
>> value: invalid date '2010/5/14 09:23:52''. [astropy.wcs.wcs]
>> WARNING: FITSFixedWarning: 'celfix' made the change 'PV1_5 : Unrecognized
>> coordinate transformation parameter'. [astropy.wcs.wcs]
>>
>> Traceback (most recent call last):
>>   File "pix2sky.py", line 49, in <module>
>>     load_wcs_from_file(sys.argv[-1])
>>   File "pix2sky.py", line 36, in load_wcs_from_file
>>     world = w.wcs_pix2world(pixcrd, 1)
>>   File
>> "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/astropy-0.3.1-py2.7-macosx-10.9-x86_64.egg/astropy/wcs/wcs.py",
>> line 1159, in wcs_pix2world
>>     'output', *args, **kwargs)
>>   File
>> "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/astropy-0.3.1-py2.7-macosx-10.9-x86_64.egg/astropy/wcs/wcs.py",
>> line 1060, in _array_converter
>>     return _return_single_array(xy, origin)
>>   File
>> "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/astropy-0.3.1-py2.7-macosx-10.9-x86_64.egg/astropy/wcs/wcs.py",
>> line 1044, in _return_single_array
>>     result = func(xy, origin)
>>   File
>> "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/astropy-0.3.1-py2.7-macosx-10.9-x86_64.egg/astropy/wcs/wcs.py",
>> line 1158, in <lambda>
>>     lambda xy, o: self.wcs.p2s(xy, o)['world'],
>> astropy.wcs._wcs.InvalidTransformError: ERROR 6 in wcsset() at line 1562
>> of file cextern/wcslib/C/wcs.c:
>> PV1_5 : Unrecognized coordinate transformation parameter.
>>
>>
>> How do I proceed?
>>
>> Cheers,
>> Angelo
>>
>> Angelo Varlotta
>> Smithsonian Astrophysical Observatory
>> 60 Garden St. MS-06, Cambridge, MA 02138, USA
>> Phone: +1-617-495-7138
>> Office: B-226
>>
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>>
>
>
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