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.. _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, ax = plt.subplots(subplot_kw=dict(projection=wcs))
ax.imshow(hdu.data, origin='lower', cmap='viridis')
ax.set(xlabel='RA', ylabel='Dec')
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