split – Create a visibility subset from an existing visibility set – manipulation task

Description

Split is the general purpose program to make a new data set that is a subset or averaged form of an existing data set. General selection parameters are included, and one or all of the various data columns (DATA, LAG_DATA and/or FLOAT_DATA, MODEL_DATA and/or CORRECTED_DATA) can be selected.

Split is often used after the initial calibration of the data to make a smaller Measurement Set with only the data that will be used in further flagging, imaging and/or self-calibration. Split can average over frequency (channels) and time (integrations).

The split task uses the MSTransform framework underneath. Split also supports the Multi-MS (MMS) format as input.

Parameters

Title

Parameter	Default	Description
vis	''	Name of input visibility file
outputvis	''	Name of output visibility file
keepmms	True	If the input is a Multi-MS the output will also be a Multi-MS.
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	'corrected'	Which data column(s) to process.
keepflags	True
width	int(1)	Number of channels to average to form one output channel
timebin	'0s'	Bin width for time averaging
combine	''	Span the timebin across scan, state or both

Parameter Explanations

vis

''

Name of input visibility file

Default: none

Example: vis=’ngc5921.ms’

outputvis

''

Name of output visibility file

Default: ‘’ (same as vis)

Example: outputvis=’ngc5921_out.ms’

IMPORTANT: if a .flagversions file with the name of the output MS exist, this task will exit with an error. The user needs to rename or remove the existing flagbackup or choose a different output name for the MS.

keepmms

True

Create a Multi-MS as the output if the input is a Multi-MS.

Default: True Options: True|False

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 CASA Docs for more information on the MMS format.

NOTE: It is not possible to do time average with combine=’scan’ if the input MMS was partitioned with separationaxis=’scan’ or ‘auto’. In this case, the task will abort with an error.

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

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

''

(Sub)array number range

Default: ‘’ (all)

uvrange

''

Select data by baseline length.

Default = ‘’ (all)

Examples: uvrange=’0~1000klambda’; uvrange from 0-1000 kilo-lambda uvrange=’>4klambda’;uvranges greater than 4 kilo-lambda uvrange=’0~1000km’; uvrange in kilometers

observation

''

Select by observation ID(s)

Subparameter of selectdata=True Default: ‘’ = all

Example: observation=’0~2,4’

feed

''

Selection based on the feed

NOT IMPLEMENTED YET! Default: ‘’ = all

datacolumn

'corrected'

Which data column(s) to use for processing

(case-insensitive). Default: ‘corrected’ Options: ‘data’, ‘model’, ‘corrected’, ‘all’,’float_data’, ‘lag_data’, ‘float_data,data’, ‘lag_data,data’

Example: datacolumn=’data’

NOTE: ‘all’ = whichever of the above that are present. If the requested column does not exist, the task will exit with an error.

keepflags

True

Keep completely flagged rows instead of dropping them.

Default: True (keep completely flagged rows in the output) Options: True|False

Keepflags has no effect on partially flagged rows. All of the channels and correlations of a row must be flagged for it to be droppable, and a row must be well defined to be keepable.

IMPORTANT: Regardless of this parameter, flagged data is never included in channel averaging. On the other hand, partially flagged rows will always be included in time averaging. The average value of the flagged data for averages containing ONLY flagged data in the relevant output channel will be written to the output with the corresponding flag set to True, while only unflagged data is used on averages where there is some unflagged data with the flag set to False.

width

int(1)

Number of channels to average to form one output channel

If a list is given, each bin will apply to one spw in the selection. Default: 1 (no channel average) Options: (int)|[int]

Example: chanbin=[2,3] => average 2 channels of 1st selected spectral window and 3 in the second one.

timebin

'0s'

Bin width for time averaging

Default: ‘0s’

Bin width for time averaging. When timebin is greater than 0s, the task will average data in time. Flagged data will be included in the average calculation, unless the parameter keepflags is set to False. In this case only partially flagged rows will be used in the average.

combine

''

Let the timebin span across scan, state or both.

Default: ‘’ (separate time bins by both of the above) Options: ‘scan’, ‘state’, ‘state,scan’

State is equivalent to sub-scans. One scan may have several state ids. For ALMA MSs, the sub-scans are limited to about 30s duration each. In these cases, the task will automatically add state to the combine parameter. To see the number of states in an MS, use the msmd tool. See help msmd.

Examples:

• combine = ‘scan’; can be useful when the scan number goes up with each integration as in many WSRT MSs.

• combine = [‘scan’, ‘state’]: disregard scan and state numbers when time averaging.

• combine = ‘state,scan’; same as above.

NOTE: It is not possible to do time average with combine=’scan’ if the input MMS was partitioned with separationaxis=’scan’ or ‘auto’. In this case, the task will abort with an error.