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FITS keywords
=============
Meaning of frequency keywords
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
WSClean uses the standard ``CRVAL`` and ``CDELT`` keywords for the frequency axis. These specify the central frequency and frequency width of the output channel, respectively. Because the frequency axis is the third dimension (the default dimensions are ra, dec, frequency, polarization), this specifically means:
.. code-block:: text
CRVAL3 -- centre frequency of the output channel
CDELT3 -- width of the output channel
.. note::
CDELT3 is sometimes used differently: in image cubes with
multiple spectral images, CDELT3 is not the channel width, but the channel
distance between adjacent images. The width of a channel might be different
from the frequency distance, for example because two observations with
partially overlapping channels are imaged together. If the first
observation has a channel at 1.0 GHz of width 0.2 GHz, and the second
observation has a channel at 1.1 GHz of width 0.2 GHz, and they fall into
the same output channel, the output image will show CDELT3 = 0.3 GHz,
because it covers 0.9 GHz up to 1.2 GHz.
WSClean-specific keywords
~~~~~~~~~~~~~~~~~~~~~~~~~
WSClean adds the following non-standard keywords to its output FITS files:
.. code-block:: text
WSCVERSI -- Version string for WSClean, e.g. "2.5.1" (since WSClean 2.5).
WSCVDATE -- Version date of WSClean, e.g. "2017-05-08" (since WSClean 2.5).
WSCNWLAY -- nwlayers
WSCDATAC -- data column
WSCWEIGH -- weighting (textual description, e.g. uniform or Briggs'(0.5))
WSCGKRNL -- gridding (antialiasing) kernel size
WSCCHANS / WSCCHANE -- channel range (only when specified)
WSCTIMES / WSCTIMEE -- time range (only when specified)
WSCFIELD -- imaged field number in MS
WSCNVIS -- gridded visibility count
WSCENVIS -- effective nr of visibilities (only for natural weighting)
WSCVWSUM -- sum of visibility weight (since WSClean 2.3)
WSCIMGWG -- Weight to be used when averaging images together
WSCNORMF -- Normalization factor that was applied to the image. The factor is
useful to undo the normalization for e.g. conversion to Kelvins.
WSCNORMF is available since WSClean 2.
WSCTHRES -- Manually applied threshold
Cleaning
--------
.. code-block:: text
WSCNITER -- max number of iterations specified
WSCGAIN -- clean gain (minor iteration gain)
WSCMGAIN -- major cleaning gain
WSCNEGCM -- whether negative components were allowed to clean
WSCNEGST -- whether stopping on first negative component during clean
WSCMAJOR -- number of major iterations actually used before reaching stopping criteria
WSCMINOR -- number of minor iterations actually used before reaching stopping criteria
The actually used number of major/minor iterations are stored in ``WSCMAJOR`` and ``WSCMINOR``, whereas ``WSCMGAIN`` and ``WSCNITER`` describe the input parameters.
No longer used
--------------
.. code-block:: text
WSCSMPSF -- whether the PSF is made smaller prior to cleaning
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