regrid an MS to a new spectral window / channel structure or frame
vis -- Name of input visibility file
default: none; example: vis='ngc5921.ms'
outputvis -- Name of output measurement set (required)
default: none; example: vis='ngc5921-regridded.ms'
passall -- if False, data not meeting the selection is omitted/deleted
or flagged (if in-row); if True, data not meeting the selection
on field and spw is passed through without modification
default: False; example:
field='NGC5921'
passall=False : only data from NGC5921 is included in output MS,
no data from other fields (e.g. 1331+305) is included
passall=True : data from NGC5921 is transformed by cvel, all other
fields are passed through unchanged
field -- Select fields in mosaic. Use field id(s) or field name(s).
['go listobs' to obtain the list id's or names]
default: ''= all fields
If field string is a non-negative integer, it is assumed to
be a field index otherwise, it is assumed to be a
field name
field='0~2'; field ids 0,1,2
field='0,4,5~7'; field ids 0,4,5,6,7
field='3C286,3C295'; field named 3C286 and 3C295
field = '3,4C*'; field id 3, all names starting with 4C
spw --Select spectral window/channels
NOTE: This selects the data passed as the INPUT to mode
default: ''=all spectral windows and channels
spw='0~2,4'; spectral windows 0,1,2,4 (all channels)
spw='0:5~61'; spw 0, channels 5 to 61
spw='<2'; spectral windows less than 2 (i.e. 0,1)
spw='0,10,3:3~45'; spw 0,10 all channels, spw 3,
channels 3 to 45.
spw='0~2:2~6'; spw 0,1,2 with channels 2 through 6 in each.
spw='0:0~10;15~60'; spectral window 0 with channels
0-10,15-60
spw='0:0~10,1:20~30,2:1;2;3'; spw 0, channels 0-10,
spw 1, channels 20-30, and spw 2, channels, 1,2 and 3
selectdata -- Other data selection parameters
default: True
>>> selectdata=True expandable parameters
antenna -- Select data based on antenna/baseline
default: '' (all)
If antenna string is a non-negative integer, it is
assumed to be an antenna index, otherwise, it is
considered an antenna name.
antenna='5&6'; baseline between antenna index 5 and
index 6.
antenna='VA05&VA06'; baseline between VLA antenna 5
and 6.
antenna='5&6;7&8'; baselines 5-6 and 7-8
antenna='5'; all baselines with antenna index 5
antenna='05'; all baselines with antenna number 05
(VLA old name)
antenna='5,6,9'; all baselines with antennas 5,6,9
index numbers
timerange -- Select data based on time range:
default = '' (all); examples,
timerange = 'YYYY/MM/DD/hh:mm:ss~YYYY/MM/DD/hh:mm:ss'
Note: if YYYY/MM/DD is missing date defaults to first
day in data set
timerange='09:14:0~09:54:0' picks 40 min on first day
timerange= '25:00:00~27:30:00' picks 1 hr to 3 hr
30min on NEXT day
timerange='09:44:00' pick data within one integration
of time
timerange='>10:24:00' data after this time
scan -- Scan number range.
default: '' (all)
example: scan='1~5'
Check 'go listobs' to insure the scan numbers are in
order.
array -- Select data by (sub)array indices
default: '' (all); example:
array='0~2'; arrays 0 to 2
mode -- Frequency Specification:
NOTE: See examples below:
default: 'channel'
mode = 'channel'; Use with nchan, start, width to specify
output spw. Produces equidistant grid based on first
selected channel. See examples below.
mode = 'velocity', means channels are specified in
velocity.
mode = 'frequency', means channels are specified in
frequency.
mode = 'channel_b', alternative 'channel' mode.
Does not force an equidistant grid. Faster.
>>> mode expandable parameters
Start, width are given in units of channels, frequency
or velocity as indicated by mode
nchan -- Number of channels in output spw
default: -1 = all channels; example: nchan=3
start -- Start or end input channel (zero-based) depending on the sign of the width parameter
default=0; example: start=5
width -- Output channel width in units of the input
channel width (sign indicates whether the start parameter is lower(+) or upper(-) end of the range)
default=1; example: width=4
interpolation -- Interpolation method (linear, nearest, cubic, spline, fftshift)
default = 'linear'
examples:
spw = '0,1'; mode = 'channel'
will produce a single spw containing all channels in spw
0 and 1
spw='0:5~28^2'; mode = 'channel'
will produce a single spw made with channels
(5,7,9,...,25,27)
spw = '0'; mode = 'channel': nchan=3; start=5; width=4
will produce an spw with 3 output channels
new channel 1 contains data from channels (5+6+7+8)
new channel 2 contains data from channels (9+10+11+12)
new channel 3 contains data from channels (13+14+15+16)
spw = '0:0~63^3'; mode='channel'; nchan=21; start = 0;
width = 1
will produce an spw with 21 channels
new channel 1 contains data from channel 0
new channel 2 contains data from channel 2
new channel 21 contains data from channel 61
spw = '0:0~40^2'; mode = 'channel'; nchan = 3; start =
5; width = 4
will produce an spw with three output channels
new channel 1 contains channels (5,7)
new channel 2 contains channels (13,15)
new channel 3 contains channels (21,23)
phasecenter -- direction measure or fieldid for the mosaic center
default: '' => first field selected ; example: phasecenter=6
or phasecenter='J2000 19h30m00 -40d00m00'
restfreq -- Specify rest frequency to use for output image
default='' Occasionally it is necessary to set this (for
example some VLA spectral line data). For example for
NH_3 (1,1) put restfreq='23.694496GHz'
outframe -- output reference frame
default='' (keep original reference frame) ; example: outframe='bary'
veltype -- definition of velocity (in mode)
default = 'radio'
hanning -- if true, Hanning smooth frequency channel data to remove Gibbs ringing
==================================================================
The intent of cvel is to transform channel labels and the
visibilities to a spectral reference frame which is appropriate
for the science analysis, e.g. from TOPO to LSRK to correct for
Doppler shifts throughout the time of the observation. Naturally,
this will change the shape of the spectral feature to some extent.
According to the Nyquist theorem you should oversample a spectrum
with twice the numbers of channels to retain the shape. Based on
some tests, however, we recommend to observe with at least
3-4 times the number of channels for each significant spectral
feature (like 3-4 times the linewidth). This will minimize
regridding artifacts in cvel.
If cvel has already established the grid that is desired for the
imaging, clean should be run with exactly the same frequency/velocity
parameters as used in cvel in order to avoid additional regridding in
clean.
Hanning smoothing is optionally offered in cvel, but tests have
shown that already the regridding process itself, if it involved
a transformation from TOPO to a non-terrestrial reference frame,
implies some smoothing (due to channel interpolation) such that
Hanning smoothing may not be necessary.
1.8.0