astro py:docs

- Index
- Modules

### Navigation

- next »
- « previous |
- Astropy v5.2.1 »
- World Coordinate System (`astropy.wcs`) »
- First Example

# First Example

This example, rather than starting from a FITS header, sets WCS values programmatically, uses those settings to transform some points, and then saves those settings to a new FITS header.

    # Set the WCS information manually by setting properties of the WCS
    # object.

    import numpy as np

    from astropy import wcs
    from astropy.io import fits

    # Create a new WCS object.  The number of axes must be set
    # from the start
    w = wcs.WCS(naxis=2)

    # Set up an "Airy's zenithal" projection
    # Vector properties may be set with Python lists, or Numpy arrays
    w.wcs.crpix = [-234.75, 8.3393]
    w.wcs.cdelt = np.array([-0.066667, 0.066667])
    w.wcs.crval = [0, -90]
    w.wcs.ctype = ["RA---AIR", "DEC--AIR"]
    w.wcs.set_pv([(2, 1, 45.0)])

    # Three pixel coordinates of interest.
    # 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))

    # Now, write out the WCS object as a FITS header
    header = w.to_header()

    # header is an astropy.io.fits.Header object.  We can use it to create a new
    # PrimaryHDU and write it to a file.
    hdu = fits.PrimaryHDU(header=header)
    # Save to FITS file
    # hdu.writeto('test.fits')

Note

The members of the WCS object correspond roughly to the key/value pairs in the FITS header. However, they are adjusted and normalized in a number of ways that make performing the WCS transformation easier. Therefore, they can not be relied upon to get the original values in the header. To build up a FITS header directly and specifically, use `astropy.io.fits.Header` directly.