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Handling Beams
==============
If you are using radio data, your cubes should have some sort of beam
information included. ``spectral-cube`` handles beams using the `radio_beam
<https://github.com/radio-astro-tools/radio_beam>`_
package.
There are two ways beams can be stored in FITS files: as FITS header
keywords (``BMAJ``, ``BMIN``, and ``BPA``) or as a ``BinTableHDU``
extension. If the latter is present, ``spectral-cube`` will return
a `~spectral_cube.spectral_cube.VaryingResolutionSpectralCube` object.
For the simpler case of a single beam across all channels, the presence
of the beam allows for direct conversion of a cube with Jy/beam units
to surface brightness (K) units. Note, however, that this requires
loading the entire cube into memory!::
>>> cube.unit # doctest: +SKIP
Unit("Jy / beam")
>>> kcube = cube.to(u.K) # doctest: +SKIP
>>> kcube.unit # doctest: +SKIP
Unit("K")
Adding a Beam
-------------
If your cube does not have a beam, a custom beam can be attached given::
>>> new_beam = Beam(1. * u.deg) # doctest: +SKIP
>>> new_cube = cube.with_beam(new_beam) # doctest: +SKIP
>>> new_cube.beam # doctest: +SKIP
Beam: BMAJ=3600.0 arcsec BMIN=3600.0 arcsec BPA=0.0 deg
This is handy for synthetic observations, which initially have a point-like beam::
>>> point_beam = Beam(0 * u.deg) # doctest: +SKIP
>>> new_cube = synth_cube.with_beam(point_beam) # doctest: +SKIP
Beam: BMAJ=0.0 arcsec BMIN=0.0 arcsec BPA=0.0 deg
The cube can then be convolved to a new resolution::
>>> new_beam = Beam(60 * u.arcsec) # doctest: +SKIP
>>> conv_synth_cube = synth_cube.convolve_to(new_beam) # doctest: +SKIP
>>> conv_synth_cube.beam # doctest: +SKIP
Beam: BMAJ=60.0 arcsec BMIN=60.0 arcsec BPA=0.0 deg
Beam can also be attached in the same way for `~spectral_cube.Projection` and
`~spectral_cube.Slice` objects.
Multi-beam cubes
----------------
Varying resolution (multi-beam) cubes are somewhat trickier to work with in
general, though unit conversion is easy. You can perform the same sort of unit
conversion with `~spectral_cube.spectral_cube.VaryingResolutionSpectralCube` s
as with regular `~spectral_cube.spectral_cube.SpectralCube` s; ``spectral-cube``
will use a different beam and frequency for each plane.
You can identify channels with bad beams (i.e., beams that differ from a reference beam,
which by default is the median beam) using
`~spectral_cube.spectral_cube.VaryingResolutionSpectralCube.identify_bad_beams`
(the returned value is a mask array where ``True`` means the channel is good),
mask channels with undesirable beams using
`~spectral_cube.spectral_cube.VaryingResolutionSpectralCube.mask_out_bad_beams`,
and in general mask out individual channels using
`~spectral_cube.spectral_cube.VaryingResolutionSpectralCube.mask_channels`.
For other sorts of operations, discussion of how to deal with these cubes via
smoothing to a common resolution is in the :doc:`smoothing` document.
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