[AstroPy] problem using the WCS package

[David Berry]

On 11 June 2013 23:09, Nikta Amiri <Nikta.Amiri at colorado.edu> wrote:
> Hello all,
>
> I am trying to use WCS package in Astropy on a fits file that has NAXIS=4 in the header, to convert some coordinates in decimal degrees to pixel values. At first, I faced this error:
>
> "RuntimeError: Input array must be 2-dimensional, where the second dimension >= 4"
>
> To get around this problem, I edited the fits header and removed NAXIS 3 and 4 as well as all corresponding CDELT/CTYPE. When running WCS_world2pix I face no error, but the pixel values that I obtain are incorrect and in some cases I even get negative values for pixels. For example, I get these values after using WCS_world2pix on a fits image with 784*282 dimension:
>
> array([[   8.35828508,  101.42723769],
>        [  -2.40845355,  373.76598659],
>        [ -54.96921724,  690.64432708],
>        [ -60.6297434 ,  625.57276931],
>        [  75.34581951,  902.489706  ]])
>
>
> Could you please let me know if there is a solution to this problem? FYI, I attach the modified fits header and the lines that I removed in the original fits header below:

Your header uses a GLS projection, and has non-zero CRVAL and CROTA
values. This is slightly problematic in that modern WCS software must
convert this from a simple shift and rotation in pixel coordinates (as
specified by the old "AIPS convention" for WCS) into an oblique
projection. See section 6.1.4 in the FITS-WCS paper II
("Representations of celestial coordinates in FITS").

You say you are getting incorrect values - how are you determining the
"correct" values for this comparison? Could you post a list of the
world coordinates you are using with the corresponding correct pixel
coordinates.

BTW - what is wrong with negative pixel coordinates? It just means the
position is off the edge of the array.

David



> Thank you,
> Nikta
>
>
>
> SIMPLE  =                    T /
> BITPIX  =                  -32 /
> NAXIS   =                    2
> NAXIS1  =                  282 /
> NAXIS2  =                  784 /
> EXTEND  =                    T /Tables following main image
> BLOCKED =                    T /Tape may be blocked
> OBJECT  = 'YY'           /Source name
> TELESCOP= 'ZZ'           /
> INSTRUME= '        '           /
> OBSERVER= 'xx  '           /
> DATE-OBS= '1901-09-27'         /Obs start date YYYY-MM-DD
> DATE-MAP= '2007-02-26'         /Last processing date YYYY-MM-DD
> BSCALE  =    1.00000000000E+00 /REAL = TAPE * BSCALE + BZERO
> BZERO   =    0.00000000000E+00 /
> BUNIT   = 'KM/S    '           /Units of flux
> EPOCH   =      1.950000000E+03 /Epoch of RA DEC
> BLANK   =                   -1 / IEEE not-a-number for blanked pixels
> DATAMAX =     -1.422322839E-01 /Maximum pixel value
> DATAMIN =     -9.989158325E+02 /Minimum pixel value
> CTYPE1  = 'RA---GLS'           /
> CRVAL1  =    1.00012502670E+01 /
> CDELT1  =     -2.222222276E-03 /
> CRPIX1  =      1.360000000E+02 /
> CROTA1  =      0.000000000E+00 /
> CTYPE2  = 'DEC--GLS'           /
> CRVAL2  =    4.10008354187E+01 /
> CDELT2  =      2.222222276E-03 /
> CRPIX2  =      3.640000000E+02 /
> CROTA2  =     -3.770000076E+01 /
>
>  And the removed lines are:
> NAXIS3  =                    1 /
> NAXIS4  =                    1 /
>
> CTYPE3  = 'FREQ    '           /
> CRVAL3  =   -3.50000000000E+05 /
> CDELT3  =     -2.600757568E+03 /
> CRPIX3  =      1.435000000E+02 /
> CROTA3  =      0.000000000E+00 /
> CTYPE4  = '        '           /
> CRVAL4  =    0.00000000000E+00 /
> CDELT4  =      1.000000000E+00 /
> CRPIX4  =      0.000000000E+00 /
> CROTA4  =      0.000000000E+00 /
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