1.2.1.0 ms.cvel - Function

1.2.1 transform spectral data to different reference frame, optionally regrid the frequency channels
Description

This function permits you to transform the spectral data of your measurement set to a given reference frame. The present reference frame information in the MS is examined and the transformation performed accordingly. Since all such transformations are linear in frequency, a pure change of reference frame only affects the channel boundary definitions.

In addition, the function permits you to permanently regrid the data, i.e. reduce the channel number and/or move the boundaries using several interpolation methods (selected using parameter ”regrid_interp_meth”). The new channels are equidistant in frequency (if parameter ”regrid_quantity” is chosen to be vrad or freq, or equidistant in wavelength if parameter ”regrid_quantity” is chosen to be vopt or wave). If ”regrid_quantity” is chosen to be ”chan”, the regridding is performed by combining the existing channels, i.e. not moving but just eliminating channel boundaries where necessary.

The regridding is applied to the channel definition and all data of the MS, i.e. all columns which contain arrays whose dimensions depend on the number of channels.

The target reference frame can be set by providing the name of a standard reference frame (LSRK, LSRD, BARY, GEO, GALACTO, LGROUP, or CMB, default = no change of frame) in parameter ”outframe”. For each field given by the list of field names in the parameter ”fields” (default = all fields), the channel frequencies are transformed from their present reference frame to the one given by parameter ”outframe”.

If the regridding parameters are set, they are interpreted in the ”outframe” reference frame. The regridding is applied to the data after the reference frame transformation.


Inputs
field	Names or IDs (0-based) of the fields for which to perform the transformation, default = all fields
	allowed:	any
	Default:	variant
outframe	Name of the reference frame to transform to (LSRK, LSRD, BARY, GEO, GALACTO, LGROUP, or CMB). If no reference frame is given, the present reference frame given by the data is used, i.e. the reference frame is not changed. Transformation to TOPO is presently not offerred. If the data already is in TOPO, it can be kept that way by not setting this parameter.
	allowed:	string
	Default:
regrid_quantity	The quantity (radio velocity (m/s), optical velocity (m/s), frequency (Hz), wavelength (m), or original channels) in which the user would like to give the regridding parameters below (”regrid_center”, ”regrid_chan_width”, ”regrid_bandwidth”): vrad, vopt, freq, wave, or chan.
	allowed:	string
	Default:	freq
regrid_velo_restfrq	Required in case the value of regrid_quantity is ”vrad” or ”vopt”: Rest frequency (Hz) for the conversion of the regrid parameters ”regrid_center”, ”regrid_chan_width”, and ”regrid_quantity” to frequencies.
	allowed:	double
	Default:
regrid_interp_meth	Name of the interpolation method (NEAR, SPLINE, etc.) used in the regridding. Flagging information is combined using ”inclusive or”.
	allowed:	string
	Default:	NEAR
regrid_center	Desired center of the spectral window after regridding in the units given by ”regrid_quantity” and in the reference frame given by ”outframe”. If no value is given, the center is kept as is.
	allowed:	double
	Default:
regrid_bandwidth	Desired width of the entire spectral window after regridding in the units given by ”regrid_quantity” and in the reference frame given by ”outframe”. If no value is given or the given width is larger than the bandwidth of the data, the width will be truncated to the maximum width possible symmetrically around the value given by ”regrid_center”.
	allowed:	double
	Default:
regrid_chan_width	Desired width of the channels in the units given by ”regrid_quantity” and in the reference frame given by ”outframe”. This implies that channels will be equidistant in the unit given by ”regrid_quantity”. If no value is given and ”regrid_quantity” is vrad or freq, the function will keep the resolution as it is. If ”regrid_quantity” is vopt or wave, the total number of channels will be kept as is.
	allowed:	double
	Default:

This example reads a measurement set and transforms its spectral axis to the LSRK reference frame.

In this example assuming a multi-field MS with rows 0, 3, and 4 present in the FIELD table, all spectral windows of these fields will be transformed to the BARY reference frame and then be regridded such that the center of the new spectral window is at radio velocity = 73961800. m/s (BARY). If the bandwidth of the observation is large enough the total width of the spectral window will be 1000 m/s, i.e. 20 channels of width 50 m/s, 10 on each side of the given center.

In this example the channels are regridded such that they are equidistant in optical velocity. The reference frame and number of channels is kept as is.

In this example, the channels are regridded such that the new bandwidth is 102 of the original channels centered on the original channel 64, and the new channels are twice as wide as the original channels.