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NRAO Home > CASA > CASA Task Reference Manual |
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0.1.18 cvel2
Requires:
Synopsis
Regrid an MS or MMS to a new spectral window, channel structure or frame
Description
The intent of cvel2 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 cvel2.
If cvel2 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 cvel2 in order to avoid additional regridding in clean.
Hanning smoothing is optionally offered in cvel2, 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.
This version of cvel2 also supports Multi-MS input, in which case it will create an output Multi-MS too.
NOTE: The parameter passall is not supported in cvel2. The user may achieve the same results of passall=True by splitting out the data that will not be regridded with cvel2 and concatenate regridded and non-regridded sets at the end. In the case of Multi-MS input, the user should use virtualconcat to achieve a concatenated MMS.
Arguments
Inputs |
| ||
vis |
| Name of input Measurement set or Multi-MS.
| |
| allowed: | string |
|
| Default: |
| |
outputvis |
| Name of output Measurement Set or Multi-MS.
| |
| allowed: | string |
|
| Default: |
| |
keepmms |
| If the input is a Multi-MS the output will also be a
Multi-MS. | |
| allowed: | bool |
|
| Default: | True |
|
passall |
| HIDDEN parameter. Pass through (write to output MS)
non-selected data with no change
| |
| allowed: | bool | |
| Default: | False |
|
field |
| Select field using ID(s) or name(s).
| |
| allowed: | any |
|
| Default: | variant
|
|
spw |
| Select spectral window/channels.
| |
| allowed: | any |
|
| Default: | variant
|
|
scan |
| Select data by scan numbers.
| |
| allowed: | any |
|
| Default: | variant
|
|
antenna |
| Select data based on antenna/baseline.
| |
| allowed: | any |
|
| Default: | variant
|
|
correlation |
| Correlation: ” ==> all, correlation=”XX,YY”.
| |
| allowed: | any |
|
| Default: | variant
|
|
timerange |
| Select data by time range.
| |
| allowed: | any |
|
| Default: | variant
|
|
intent |
| Select data by scan intent.
| |
| allowed: | any |
|
| Default: | variant
|
|
array |
| Select (sub)array(s) by array ID number.
| |
| allowed: | any |
|
| Default: | variant
|
|
uvrange |
| Select data by baseline length.
| |
| allowed: | any |
|
| Default: | variant
|
|
observation |
| Select by observation ID(s).
| |
| allowed: | any |
|
| Default: | variant
|
|
feed |
| Multi-feed numbers: Not yet implemented.
| |
| allowed: | any |
|
| Default: | variant
|
|
datacolumn |
| Which data column(s) to process.
| |
| allowed: | string |
|
| Default: | all |
|
mode |
| Regridding mode
(channel/velocity/frequency/channel_b).
| |
| allowed: | string |
|
| Default: | channel |
|
nchan |
| Number of channels in the output spw (-1=all).
| |
| allowed: | int |
|
| Default: | -1 |
|
start |
| First channel to use in the output spw
(mode-dependant).
| |
| allowed: | any |
|
| Default: | variant 0 |
|
width |
| Number of input channels that are used to create an
output channel.
| |
| allowed: | any |
|
| Default: | variant 1 |
|
interpolation |
| Spectral interpolation method
| |
| allowed: | string |
|
| Default: | linear |
|
phasecenter |
| Image phase center: position or field index
| |
| allowed: | any |
|
| Default: | variant
|
|
restfreq |
| Rest frequency to use for output.
| |
| allowed: | string |
|
| Default: |
|
|
outframe |
| Output reference frame (”=keep input frame).
| |
| allowed: | string |
|
| Default: |
|
|
veltype |
| Velocity definition.
| |
| allowed: | string |
|
| Default: | radio |
|
hanning |
| Hanning smooth data to remove Gibbs ringing.
| |
| allowed: | bool |
|
| Default: | False |
|
void
Example
Detailed description of keyword arguments:
--- Input/Output parameters ---
vis -- Name of input visibility file
default: ’’; example: vis=’ngc5921.ms’
outputvis -- Name of output visibility file or Multi-MS
default: ’’; example: outputvis=’ngc5921.mms’
keepmms -- Create a Multi-MS as the output if the input is a Multi-MS.
default: True
By default it will create a Multi-MS when the input is a Multi-MS.
The output Multi-MS will have the same partition axis of the input MMS.
See ’help partition’ for more information on the MMS format.
NOTE: It is not possible to combine the spws if the input MMS was partitioned with
separationaxis=’spw’. In this case, the task will abort with an error.
--- Data selection parameters ---
field -- Select field using field id(s) or field name(s).
[run listobs to obtain the list iof d’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’; fields named 3C286 and 3C295
field = ’3,4C*’; field id 3, all names starting with 4C
spw -- Select spectral window/channels
default: ’’=all spectral windows and channels
spw=’0~2,4’; spectral windows 0,1,2,4 (all channels)
spw=’<2’; spectral windows less than 2 (i.e. 0,1)
spw=’0:5~61’; spw 0, channels 5 to 61
spw=’0,10,3:3~45’; spw 0,10 all channels, spw 3 - chans 3 to 45.
spw=’0~2:2~6’; spw 0,1,2 with channels 2 through 6 in each.
spw = ’*:3~64’ channels 3 through 64 for all sp id’s
spw = ’ :3~64’ will NOT work.
NOTE: mstransform does not support multiple channel ranges per
spectral window (’;’).
scan -- Scan number range
default: ’’=all
antenna -- Select data based on antenna/baseline
default: ’’ (all)
Non-negative integers are assumed to be antenna indices, and
anything else is taken as an antenna name.
examples:
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 5
antenna=’5,6,10’: all baselines including antennas 5, 6, or 10
antenna=’5,6,10&’: all baselines with *only* antennas 5, 6, or
10. (cross-correlations only. Use &&
to include autocorrelations, and &&&
to get only autocorrelations.)
antenna=’!ea03,ea12,ea17’: all baselines except those that
include EVLA antennas ea03, ea12, or
ea17.
correlation -- Correlation types or expression.
default: ’’ (all correlations)
example: correlation=’XX,YY’
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, timerange defaults to the
first day in the dataset
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’ data within one integration of time
timerange=’>10:24:00’ data after this time
array -- (Sub)array number range
default: ’’=all
uvrange -- Select data within uvrange (default units meters)
default: ’’=all; example:
uvrange=’0~1000klambda’; uvrange from 0-1000 kilo-lambda
uvrange=’>4klambda’;uvranges greater than 4 kilo-lambda
uvrange=’0~1000km’; uvrange in kilometers
observation -- Select by observation ID(s)
default: ’’=all
feed -- Selection based on the feed - NOT IMPLEMENTED YET
default: ’’=all
datacolumn -- Which data column to use for processing (case-insensitive).
default: ’all’;
options: ’data’, ’model’, ’corrected’, ’all’,’float_data’, ’lag_data’,
’float_data,data’, ’lag_data,data’.
example: datacolumn=’data’
NOTE: ’all’ = whichever of the above that are present. If the requested
column does not exist, the task will exit with an error.
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. To be used in mosaics to indicate
the center direction to be used in the spectral coordinate transformation.
default: ’’ (first selected field)
options: FIELD_ID (int) or center coordinate measure (str).
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 (not case-sensitive)
possible values: LSRK, LSRD, BARY, GALACTO, LGROUP, CMB, GEO, TOPO, or SOURCE
(SOURCE is meant for solar system work and corresponds to GEO + radial velocity
correction for ephemeris objects).
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 cvel2 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 cvel2.
If cvel2 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 cvel2 in order to avoid additional regridding in
clean.
Hanning smoothing is optionally offered in cvel2, 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.
More information about CASA may be found at the
CASA web page
Copyright © 2016 Associated Universities Inc., Washington, D.C.
This code is available under the terms of the GNU General Public Lincense
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