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, tclean 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	`''`	Name of input visibility file
outputvis	`''`	Name of output visibility file
keepmms	`True`	Create a Multi-MS as the output if the input is a Multi-MS
passall	`False`	Hidden parameter
field	`''`	Select field using field id(s) or field name(s)
spw	`''`	Select spectral window/channels
scan	`''`	Scan number range
antenna	`''`	Select data based on antenna/baseline
correlation	`''`	Select data based on correlation
timerange	`''`	Select data based on time range
intent	`''`	Select observing 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'`	Data column(s) to process.
mode	`'channel'`	Regridding mode (channel/velocity/frequency/channel_b).
nchan	`int(-1)`	Number of channels in the output spw
start	`int(0)`	First input channel to use
width	`int(1)`	Channel width of the output visibilities.
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 to use for output.
outframe	`''`	Output reference frame.
veltype	`'radio'`	Velocity definition.
hanning	`False`	Hanning smooth data to remove Gibbs ringing.

Parameter Explanations

vis

''

Name of input visibility file: Default: none

Example: vis=’ngc5921.ms’

outputvis

''

Name of output visibility file or Multi-MS: Default: none

Example: vis=’ngc5921_out.ms’

keepmms

True

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

passall

False

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

field

''

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

Default: ‘’ (all fields)

Use ‘go listobs’ to obtain the list id’s or names. If field string is a non-negative integer, it is assumed a field index, otherwise, it is assumed a field name.

Examples: 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

Default: ‘’=all spectral windows and channels

Examples: 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: Subparameter of selectdata=True default: ‘’ = all

antenna

''

Select data based on antenna/baseline

Subparameter of selectdata=True default: ‘’ (all)

If antenna string is a non-negative integer, it is assumed an antenna index, otherwise, it is assumed 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 with indices 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=’5,6,10’; all baselines with antennas 5,6,10 index numbers antenna=’!ea03,ea12,ea17’: all baselines except those that include EVLA antennas ea03, ea12, or ea17.

correlation

''

Select data based on correlation: Default: ‘’ (all)

Example: correlation=’XX,YY’.

timerange

''

Select data based on time range: Subparameter of selectdata=True 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

intent

''

Select observing intent: Default: ‘’ (no selection by intent)

Example: intent=’BANDPASS’ (selects data labelled with BANDPASS intent)

array

''

Select (sub)array(s) by array ID number.: Default = ‘’ (all)

uvrange

''

Select data by baseline length.

observation

''

Select by observation ID(s): Subparameter of selectdata=True Default: ‘’ = all

Example: observation=’0~2,4’

feed

''

Multi-feed numbers: Not yet implemented.

datacolumn

'all'

Which data column(s) to process.

mode

'channel'

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

Default: ‘channel’ Options: ‘channel’, ‘velocity’, ‘frequency’, ‘channel_b’

mode = ‘channel’; Use with nchan, start, width to specify output spw. Produces equidistant grid based on first selected channel.
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.

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)

nchan

int(-1)

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

Subparameter of mode=’channel|velocity|frequency|channel_b’ Default: -1 = all channels

Used for regridding, together with ‘start’ and ‘width’.

Example: nchan=3

start

int(0)

Start or end input channel (zero-based), depending on the sign of the width parameter

Subparameter of mode=’channel|velocity|frequency|channel_b’

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.

Subparameter of mode=’channel|velocity|frequency|channel_b’

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.

Note: the sign indicates whether the start parameter is lower(+) or upper(-) end of the range.

interpolation

'linear'

Spectral interpolation method: Subparameter of mode=’channel|velocity|frequency|channel_b’ Default = ‘linear’ Options: linear, nearest, cubic, spline, fftshift

phasecenter

''

Phase center direction to be used for the spectral coordinate transformation.

Default: ‘’ (first selected field) Options: FIELD_ID (int) or center coordinate measure (str).

Phase direction measure or fieldid. To be used in mosaics to indicate the center direction to be used in the spectral coordinate transformation.

Examples: phasecenter=6 phasecenter=’J2000 19h30m00 -40d00m00’

restfreq

''

Rest frequency to use for output visibilities.

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).

Default: ‘’ (keep original reference frame) Options: 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.

Example: outframe=’BARY’

veltype

'radio'

Definition of velocity (in mode): Default = ‘radio’

hanning

False

Hanning smooth data to remove Gibbs ringing.: Default: False Options: False|True