cvel

cvel(vis, outputvis='', passall=False, field='', spw='', selectdata=True, antenna='', timerange='', scan='', array='', mode='channel', nchan=- 1, start='0', width='1', interpolation='linear', phasecenter='', restfreq='', outframe='', veltype='radio', hanning=False)[source]

regrid an MS to a new spectral window / channel structure or frame

[Description] [Examples] [Development] [Details]

Parameters

  • vis (string) - Name of input measurement set

  • outputvis (string=’’) - Name of output measurement set

  • passall (bool=False) - Pass through (write to output MS) non-selected data with no change

  • field ({string, stringArray, int, intArray}=’’) - Select field using field id(s) or field name(s)

  • spw ({string, stringArray, int, intArray}=’’) - Select spectral window/channels

  • selectdata (bool=True) - Other data selection parameters

    selectdata = True

    • timerange (string=’’) - Range of time to select from data

    • array (string=’’) - (sub)array indices

    • antenna (string=’’) - Select data based on antenna/baseline

    • scan (string=’’) - scan number range

  • mode (string=’channel’) - Regridding mode

    mode = channel

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

    • start (variant=’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 (variant=’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 (string=’linear’) - Spectral interpolation method

    mode = channel_b

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

    • start (variant=’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 (variant=’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 (string=’linear’) - Spectral interpolation method

    mode = velocity

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

    • start (variant=’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 (variant=’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 (string=’linear’) - Spectral interpolation method

    mode = frequency

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

    • start (variant=’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 (variant=’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 (string=’linear’) - Spectral interpolation method

  • phasecenter (variant=’’) - Phase center direction to be used for the spectral coordinate transformation: direction measure or field index

  • restfreq (string=’’) - rest frequency (see help)

  • outframe (string=’’) - Output frame (not case-sensitive, ''=keep input frame)

  • veltype (string=’radio’) - velocity definition

  • hanning (bool=False) - If true, Hanning smooth data before regridding to remove Gibbs ringing.

Description

Warning

ALERT: The task cvel will soon be replaced by the functionality (and underlying code) currently offered by cvel2. cvel2 is using the mstransform framework for optimal performance and will be renamed cvel after complete validation of all modes in the near future. See below and also the discussion on the cvel2 task page for differences between cvel and cvel2.

VLA and ALMA datasets are observed in TOPOcentric velocity frames with fixed sky frequencies that are calculated from the observatory velocity at the start of the observation, and kept throughout (Doppler setting). The data will need to be regridded to a constant velocity grid to avoid a smearing of spectral features (e.g. to the LSRK or BARYcentric velocity frames, see Spectral Line Imaging). cvel can perform this operation. We advise that for spectral regridding to a standard velocity system like LSRK or BARY the expected spectral features are oversampled at least by a factor of 3-4 of the linewidth to preserve the spectral shape and to avoid regridding artifacts.

cvel is in fact a more general tasks that transforms visibilities between spectral frames for visibilities. Doppler correction is also applied during imaging with tclean, which is recommended for most cases. cvel is still useful if the MS itself needs to be stored in a specific frame, e.g. for self-calibration on fixed velocity channels. An MS that was regridded using cvel can be imaged in channel mode in tclean (although tclean will perform some internal regridding anyways).

Gridding modes and parameters

cvel offers four gridding mode s: ‘channel’, ‘velocity’, ‘frequency’, and ‘channel_b’. All of the modes have the same four subparameters nchan, start, width, and interpolation. nchan is the number of channels for all modes.The meaning and units of the parameter start and width depend on the gridding mode used:

  • For the modes ‘channel’ and ‘channel_b’, start is the input starting channel, and width the number of input channels to be merged into a single output channel.

  • In velocity and frequency mode, start defines the first channel of the output grid and width the width of each channel in the output in velocity or frequency units, respectively.

Negative width numbers run the channel sequence in the opposite direction.

The difference between ‘channel’ and ‘channel_b’ is that ‘channel’ forces the output to be on an equidistant grid based on first selected channel, whereas ‘channel_b’ does not (which improves the speed of cvel). Mode ‘velocity’ also requires the specification of a rest frequency (restfreq parameter, and definition type (‘radio’ or ‘optical’) in the veltype parameter. See also the relevant sections of the Spectral Line Imaging and Spectral Frames chapters.

interpolation specifies the interpolation method between the spectral channels. The interpolation method ‘fftshift’ calculates the transformed visibilities by applying a FFT, then a phase ramp, and then an inverse FFT. It will also perform pre-averaging, if necessary for the requested output grid (this will also increase the S/N). Note that if you want to use this interpolation method, your frequency grid needs to be equidistant, i.e. it only works in mode velocity with veltype=’radio’, in mode ‘frequency’, and in mode ‘channel’ (in the latter only if the input grid is itself equidistant in frequency). Note also that, as opposed to all other interpolation methods, this method will apply a constant (frequency independent) shift in frequency which is not fully correct in the case of large fractional bandwidth of the given spectral window

The phasecenter parameter can be used to specify the field id or position that is used for the transformations. This can be useful for larger mosaics.

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.

cvel and cvel2

Development of cvel has stopped. For a more up to date version please see task cvel2. The regridding calculations of cvel2 have been modified in order to better align it with regridding in tclean. Also, cvel2 will not perform a pre-average step automatically when the width of the output channels is more than twice the widtch of the input channels.

Examples

**Example 1: **

Regrid ‘myMS.ms’ to a new ‘myMSregridded.ms’ using veolicty mode and a LSRK radio velocity definition. The output data has a structure of 10 channels, starting at 123 km/s with a width of 0.1km/s. We use the HI rest frequency of 1.420405752 GHz.

cvel(vis='myMS.ms', outputvis='myMSregridded.ms',
outframe='LSRK', mode='velocity', veltype='radio',
restfreq='1.420405752GHz', nchan=10, start='123km/s',
width='0.1km/s')

**Example 2: **

Regrid the same MS, but this time using channel mode. We start at channel 5, and create 10 new output channels, grouping 7 channels in the new bins. We also run the output MS in reverse spectral order (note the negative value of width). This time we request a BARYcentric frame and use the interpolation method ‘fftshift’.

cvel(vis='myMS.ms', outputvis='myMSregridded.ms',
outframe='BARY',  mode='channel', nchan=10, start=5, width=-7,
interpolation='fftshift')
Development

No additional development details

Parameter Details

Detailed descriptions of each function parameter

vis (string) - Name of input measurement set
outputvis (string='') - Name of output measurement set
passall (bool=False) - Pass through (write to output MS) non-selected data with no change
field ({string, stringArray, int, intArray}='') - Select field using field id(s) or field name(s)
spw ({string, stringArray, int, intArray}='') - Select spectral window/channels
selectdata (bool=True) - Other data selection parameters
antenna (string='') - Select data based on antenna/baseline
timerange (string='') - Range of time to select from data
scan (string='') - scan number range
array (string='') - (sub)array indices
mode (string='channel') - Regridding mode
nchan (int=-1) - Number of channels in output spw (-1=all). Used for regridding, together with 'start' and 'width'.
start (variant='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 (variant='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 (string='linear') - Spectral interpolation method
phasecenter (variant='') - Phase center direction to be used for the spectral coordinate transformation: direction measure or field index
restfreq (string='') - rest frequency (see help)
outframe (string='') - Output frame (not case-sensitive, ''=keep input frame)
veltype (string='radio') - velocity definition
hanning (bool=False) - If true, Hanning smooth data before regridding to remove Gibbs ringing.