Description

concatenate several visibility data sets.

This task concatenates several visibility data sets into a single MeasurementSet (MS).

 

Input and output MeasurementSets

The list of data sets given in the vis argument are chronologically concatenated into an output data set (named in concatvis), i.e. the data sets in vis are first ordered by the time of their earliest integration and then concatenated.

For example, vis = ['src2.ms','ngc5921.ms','ngc315.ms'], concatvis = 'src2.ms'

If concatvis already exists (e.g., it is the same as the first input data set), then the other input data sets will be appended to the concatvis data set. There is no limit to the number of input data sets. If none of the input data sets has any scratch columns (model or corrected columns), none is created in the concatvis. Otherwise these columns are created on output and initialized to their default value (1 in model column, data in corrected column) for those data with no input scratch columns. 

NOTE: If the concatvis output file exits on disk then the input files are added to this file. Otherwise the new file contains the concatenated data. Be careful here when concatenating to an existing file.

 

Spectral and position shift tolerances

Spectral windows for each data set with the same channelization (equal numbers of channels), and within a specified frequency tolerance (parameter freqtol) of another data set, will be combined into one spectral window. By default, freqtol = '', which is understood by CASA to be a frequency tolerance of 1 Hz. For example, freqtol = '10MHz' will not combine spectral windows unless they are within 10 MHz of each other. 

NOTE: This option is useful to combine spectral windows with very slight frequency differences caused by Doppler tracking, for example.

A field position in one data set that is within a specified direction tolerance (parameter dirtol) of another field position in any other data set will be combined into one field. The field names need not be the same -- only their position is used. In these cases, the actual direction and field name in the resulting, merged output field will be the ones from the chronologically-first input MS. Each appended dataset is assigned a new observation ID (provided the entries in the observation table are indeed different). By default, dirtol = '', which is understood by CASA to be a position tolerance of 1 milliarcsec (mas). For example, dirtol = '1arcsec' will not combine fields unless their phase centers differ by less than 1 arcsec. If the field names are different in the input data sets, the name in the output data set will be the first relevant data set in the list. Use the parameter respectname = True to indicate that fields with a different name should not be merged even if their direction agrees (within dirtol). 

NOTE: There is no constraint on data that is simultaneously observed for more than one field; for example multi-source correlation of VLBA data.

 

Operations performed on output MeasurementSets 

Use the parameter timesort = True to sort the output visibility table in time. 

Set copypointing = True (the default behavior) to make a proper copy of the POINTING subtable (though this can be time-consuming). If False, the result is an empty POINTING table.

The weights of the individual MSs will be scaled in the concatenated output MS by the factors in the list given in visweightscale. SIGMA will be scaled by 1/sqrt(factor). This parameter is useful for handling heterogeneous arrays. Use plotms to inspect the "Wt" columns as reference for determining the scaling factors. For example, visweightscale = [1.,3.,3.] will scale the weights of the second and third MS by a factor 3 and the SIGMA column of these MSs by a factor 1/sqrt(3).

By default, concat will only merge two ephemeris fields if the first ephemeris covers the time range of the second. Otherwise, two separate fields with separate ephemerides are placed in the output MS. In order to override this behaviour and force concat to merge the non-overlapping or only partially overlapping input ephemerides, the name or ID of the field in question needs to be placed into the list in parameter forcesingleephemfield. For example, forcesingleephemfield = ['Neptune'] will make sure that there is only one joint ephemeris for field Neptune in the output MS.