[AstroPy] problem using the WCS package
David Berry
d.berry@jach.hawaii....
Wed Jun 12 05:22:11 CDT 2013
On 11 June 2013 23:09, Nikta Amiri <Nikta.Amiri@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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