Loading WCS Information from a FITS File

This example loads a FITS file (supplied on the command line) and uses the FITS keywords in its primary header to create a WCS and transform.

# Load the WCS information from a fits header, and use it
# to convert pixel coordinates to world coordinates.

import numpy as np
from astropy import wcs
from astropy.io import fits
import sys


def load_wcs_from_file(filename):
    # Load the FITS hdulist using astropy.io.fits
    hdulist = fits.open(filename)

    # Parse the WCS keywords in the primary HDU
    w = wcs.WCS(hdulist[0].header)

    # Print out the "name" of the WCS, as defined in the FITS header
    print(w.wcs.name)

    # Print out all of the settings that were parsed from the header
    w.wcs.print_contents()

    # Three pixel coordinates of interest.
    # Note we've silently assumed an NAXIS=2 image here.
    # The pixel coordinates are pairs of [X, Y].
    # The "origin" argument indicates whether the input coordinates
    # are 0-based (as in Numpy arrays) or
    # 1-based (as in the FITS convention, for example coordinates
    # coming from DS9).
    pixcrd = np.array([[0, 0], [24, 38], [45, 98]], dtype=np.float64)

    # Convert pixel coordinates to world coordinates
    # The second argument is "origin" -- in this case we're declaring we
    # have 0-based (Numpy-like) coordinates.
    world = w.wcs_pix2world(pixcrd, 0)
    print(world)

    # Convert the same coordinates back to pixel coordinates.
    pixcrd2 = w.wcs_world2pix(world, 0)
    print(pixcrd2)

    # These should be the same as the original pixel coordinates, modulo
    # some floating-point error.
    assert np.max(np.abs(pixcrd - pixcrd2)) < 1e-6

    # The example below illustrates the use of "origin" to convert between
    # 0- and 1- based coordinates when executing the forward and backward
    # WCS transform.
    x = 0
    y = 0
    origin = 0
    assert (w.wcs_pix2world(x, y, origin) ==
            w.wcs_pix2world(x + 1, y + 1, origin + 1))


if __name__ == '__main__':
    load_wcs_from_file(sys.argv[-1])