Continuum fitting and subtraction in the uv plane
Continuum fitting and subtraction in the uv plane:
This task estimates the continuum emission by fitting polynomials to
the real and imaginary parts of the spectral windows and channels
selected by fitspw. This fit represents a model of the continuum in
all channels.
The fitted continuum spectrum is subtracted from all channels
selected in spw, and the result (presumably only line emission)
is stored in a new MS (vis + ".contsub").
It will read from the CORRECTED_DATA column of vis if it is present,
or DATA if it is not. Whichever column is read is presumed to have
already been calibrated.
If want_cont is True, the continuum fit is placed in a second new MS
(vis + '.cont'). N.B. because the continuum model is necessarily a
smoothed fit, images made with it are liable to have their field of
view reduced in some strange way. Images of the continuum should be
made by simply excluding the line channels (and probably averaging the
remaining ones) in clean.
Keyword arguments:
vis -- Name of input visibility file
default: none; example: vis='ngc5921.ms'
field -- Field selection for continuum estimation and subtraction.
The estimation and subtraction is done for each selected field
in turn. (Run listobs to get lists of the ID and names.)
default: field = '' means select all fields
field = 1 # will get field_id=1 (if you give it an
integer, it will retrieve the source with that index.
field = '1328+307' specifies source '1328+307'
field = '13*' will retrieve '1328+307' and any other fields
beginning with '13'
fitspw -- Selection of spectral windows and channels to use in the
fit for the continuum, using general spw:chan syntax.
See the note under combine.
default: '' (all)
example: fitspw='0:5~30;40~55'
combine -- Data axes to combine for the continuum estimate.
It must include 'spw' if spw contains spws that are not in
fitspw!
default: '' --> solutions will break at scan, field, and spw
boundaries according to solint
Options: '', 'spw'', 'scan', or 'spw, scan'
example: combine='spw' --> form spw-merged continuum estimate
solint -- Timescale for per-baseline fit (units optional)
default (recommended): 'int' --> no time averaging, do a
fit for each integration and let the
noisy fits average out in the image.
example: solint='10s' --> average to 10s before fitting
10 or '10' --> '10s' (unitless: assumes seconds)
options: 'int' --> per integration
'inf' --> per scan
If solint is longer than 'int', the continuum estimate can be
corrupted by time smearing!
fitorder -- Polynomial order for the fits of the continuum w.r.t.
frequency. fitorders > 1 are strongly discouraged
because high order polynomials have more flexibility, may
absorb line emission, and tend go wild at the edges of
fitspw, which is not what you want.
default: 0 (constant); example: fitorder=1
spw -- Optional per spectral window selection of channels to include
in the output. See the note under combine.
The spectral windows will be renumbered to start from 0, as in
split.
want_cont -- Create vis + '.cont' to hold the continuum estimate.
default: 'False'; example: want_cont=True
The continuum estimate will be placed in vis + '.cont'
async -- Run task in a separate process (return CASA prompt)
default: False; example: async=True
1.8.0