cvel2 – Regrid an MS or MMS to a new spectral window, channel structure or frame – manipulation task

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.

Parameters

Title
Parameter	Default	Description
vis	`''`
outputvis	`''`
keepmms	`True`
passall	`False`
field	`''`
spw	`''`
scan	`''`
antenna	`''`
correlation	`''`
timerange	`''`
intent	`''`
array	`''`
uvrange	`''`
observation	`''`
feed	`''`
datacolumn	`'all'`
mode	`'channel'`
nchan	`int(-1)`
start	`int(0)`
width	`int(1)`
interpolation	`'linear'`
phasecenter	`''`
restfreq	`''`
outframe	`''`
veltype	`'radio'`
hanning	`False`

Parameter Explanations

vis

''

Name of input Measurement set or Multi-MS.

outputvis

''

Name of output Measurement Set or Multi-MS.

keepmms

True

If the input is a Multi-MS the output will also be a Multi-MS.

passall

False

HIDDEN parameter. Pass through (write to output MS) non-selected data with no change

field

''

Select field using ID(s) or name(s).

spw

''

Select spectral window/channels.

scan

''

Select data by scan numbers.

antenna

''

Select data based on antenna/baseline.

correlation

''

Correlation: ‘’ ==> all, correlation=”XX,YY”.

timerange

''

Select data by time range.

intent

''

Select data by scan intent.

array

''

Select (sub)array(s) by array ID number.

uvrange

''

Select data by baseline length.

observation

''

Select by observation ID(s).

feed

''

Multi-feed numbers: Not yet implemented.

datacolumn

'all'

Which data column(s) to process.

mode

'channel'

Regridding mode (channel/velocity/frequency/channel_b).

nchan

int(-1)

Number of channels in the output spw (-1=all).

start

int(0)

First channel to use in the output spw (mode-dependant).

width

int(1)

Number of input channels that are used to create an output channel.

interpolation

'linear'

Spectral interpolation method

phasecenter

''

Image phase center: position or field index

restfreq

''

Rest frequency to use for output.

outframe

''

Output reference frame (‘’=keep input frame).

veltype

'radio'

Velocity definition.

hanning

False

Hanning smooth data to remove Gibbs ringing.