.. _wcstools:

Subsetting and Pixel Scales
^^^^^^^^^^^^^^^^^^^^^^^^^^^

WCS objects can be broken apart into their constituent axes using the
`~astropy.wcs.WCS.sub` function.  There is also a `~astropy.wcs.WCS.celestial`
convenience function that will return a WCS object with only the celestial axes
included.

The pixel scales of a celestial image or the pixel dimensions of a non-celestial
image can be extracted with the utility functions
`~astropy.wcs.utils.proj_plane_pixel_scales` and
`~astropy.wcs.utils.non_celestial_pixel_scales`. Likewise, celestial pixel
area can be extracted with the utility function
`~astropy.wcs.utils.proj_plane_pixel_area`.

Matplotlib plots with correct WCS projection
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The :ref:`WCSAxes <wcsaxes>` framework, previously a standalone package, allows
the :class:`~astropy.wcs.WCS` to be used to define projections in Matplotlib.
More information on using WCSAxes can be found :ref:`here <wcsaxes>`.

.. plot::
    :context: reset
    :include-source:
    :align: center

    import warnings
    from matplotlib import pyplot as plt
    from astropy.io import fits
    from astropy.wcs import WCS, FITSFixedWarning
    from astropy.utils.data import get_pkg_data_filename

    filename = get_pkg_data_filename('tutorials/FITS-images/HorseHead.fits')

    hdu = fits.open(filename)[0]
    with warnings.catch_warnings():
        # Ignore a warning on using DATE-OBS in place of MJD-OBS
        warnings.filterwarnings('ignore', message="'datfix' made the change",
                                category=FITSFixedWarning)
        wcs = WCS(hdu.header)

    fig = plt.figure()
    fig.add_subplot(111, projection=wcs)
    plt.imshow(hdu.data, origin='lower', cmap=plt.cm.viridis)
    plt.xlabel('RA')
    plt.ylabel('Dec')