Find physical coordinates of a pixel in a fits file with python

Question

I am tying to get the physical sky coordinates of a given pixel from within a python script. I would like to use astropy's WCS, but I'll do anything from within python.

I have tried these two snippets of code.

from astropy.io import fits
from astropy.wcs import WCS

def astropymethod1(img):
    # from http://astropy.readthedocs.org/en/latest/wcs/
    w = WCS(img)
    lon, lat = w.all_pix2world( 100., 100., 1)
    print lon, lat

def astropymethod2(img):
    # from http://astropy.readthedocs.org/en/latest/wcs/
    hdu = fits.open(img)
    w = WCS(hdu[0].header)
    lon, lat = w.wcs_pix2world(100., 100., 1)
    print lon, lat

The issues are I get an error the first time I try to use WCS and the result is only ever the pixel values I put in.

WARNING: FITSFixedWarning: The WCS transformation has more axes (2) than the image it is associated with (0) [astropy.wcs.wcs]

Answer 1

The problem is that you have a multi-extension FITS file. Here's an example session showing how you can get access to the appropriate WCS:

In [1]: from astropy.io import fits

In [2]: h = fits.getheader('SN1415_F625W_1_drz.fits')

In [3]: f = fits.open('SN1415_F625W_1_drz.fits')

In [4]: f
Out[4]:
[<astropy.io.fits.hdu.image.PrimaryHDU at 0x106735490>,
 <astropy.io.fits.hdu.image.ImageHDU at 0x106749750>,
 <astropy.io.fits.hdu.image.ImageHDU at 0x106751310>,
 <astropy.io.fits.hdu.image.ImageHDU at 0x106751d10>,
 <astropy.io.fits.hdu.table.BinTableHDU at 0x1067dfdd0>]

In [5]: from astropy import wcs

In [6]: w = wcs.WCS(f[0].header)
WARNING: FITSFixedWarning: The WCS transformation has more axes (2) than the image it is associated with (0) [astropy.wcs.wcs]

In [7]: w.wcs.naxis
Out[7]: 2

In [8]: f[0].data

In [9]: w = wcs.WCS(f[1].header)

In [10]: w.wcs.naxis
Out[10]: 2

In [11]: f[1].data
Out[11]:
array([[ 0.01986978, -0.04018363,  0.03330525, ...,  0.        ,
         0.        ,  0.        ],
       [ 0.0695872 , -0.00979143,  0.00147662, ...,  0.        ,
         0.        ,  0.        ],
       [-0.09292094,  0.02481506, -0.01057338, ...,  0.        ,
         0.        ,  0.        ],
       ...,
       [ 0.        ,  0.        ,  0.        , ...,  0.02375774,
         0.0389731 ,  0.03825707],
       [ 0.        ,  0.        ,  0.        , ..., -0.01570918,
        -0.01053802,  0.00461219],
       [ 0.        ,  0.        ,  0.        , ..., -0.0638448 ,
        -0.0240754 ,  0.02679451]], dtype=float32)

In [12]: w.wcs_pix2world(100., 100., 1)
Out[12]: [array(6.113076380801787), array(0.616758775753701)]

So you probably want to redefine your method:

def astropymethod2(img, hduid=1):
    # from http://astropy.readthedocs.org/en/latest/wcs/
    hdu = fits.open(img)
    w = WCS(hdu[hduid].header)
    lon, lat = w.wcs_pix2world(100., 100., 1)
    print lon, lat