cvel – regrid an MS to a new spectral window / channel structure or frame – manipulation task

Description

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.

Parameters

Title

Parameter	Default	Description
vis	''
outputvis	''
passall	False
field	''
spw	''
selectdata	True
antenna	''
timerange	''
scan	''
array	''
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

outputvis

''

Name of output measurement set

passall

False

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

field

''

Select field using field id(s) or field name(s)

spw

''

Select spectral window/channels

selectdata

True

Other data selection parameters

antenna

''

Select data based on antenna/baseline

timerange

''

Range of time to select from data

scan

''

scan number range

array

''

(sub)array indices

mode

'channel'

Regridding mode

nchan

int(-1)

Number of channels in output spw (-1=all). Used for regridding, together with 'start' and 'width'.

start

int(0)

Start of the output visibilities. Used for regridding, together with 'width' and 'nchan'. It can be in different units, depending on the regridding mode: first input channel (mode='channel'), first velocity (mode='velocity'), or first frequency (mode='frequency'). Example values: '5', '0.0km/s', '1.4GHz', for channel, velocity, and frequency modes, respectively.

width

int(1)

Channel width of the output visibilities. Used for regridding, together with 'start', and 'nchan'. It can be in different units, depending on the regridding mode: number of input channels (mode='channel'), velocity (mode='velocity'), or frequency (mode='frequency'. Example values: '2', '1.0km/s', '1.0kHz', for channel, velocity, and frequency modes, respectively.

interpolation

'linear'

Spectral interpolation method

phasecenter

''

Phase center direction to be used for the spectral coordinate transformation: direction measure or field index

restfreq

''

rest frequency (see help)

outframe

''

Output frame (not case-sensitive, ''=keep input frame)

veltype

'radio'

velocity definition

hanning

False

If true, Hanning smooth data before regridding to remove Gibbs ringing.