1.3.1.0 ms.tofits

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ms.tofits - Function

1.3.1 Convert a measurement set to a uvfits file

Description

This function writes a uvfits file that contains the data in the measurement set associated with this tool. The fits file is always written in floating point format and the data are always stored in the primary array of the fits file.

IMPORTANT NOTE: In general, some of the data averaging features of this method have never worked properly. In general, users should run mstransform to select and average data prior to running tofits(). The associated input parameters are slowly being deprecated and removed.

If the measurement set has been imaged or calibrated in CASA, it may contain additional data columns. You need to select ONE of these columns to be written to the fits file. The possible options are:

observed: This is the raw data as collected by the telescope. All interferometric measurement sets must contain this column. A synonym for ’observed’ is ’data’.
corrected: This is the calibrated data. A synonym for ’corrected’ is ’corrected_data’.
model: This is the visibilites that would be measured using the current model of the sky. A synonym for ’model’ is ’model_data’.

The parsing of these strings is case insensitive. If any other option is specified then the observed data will be written.

By default a single-source uvfits file is written, but if the measurement set contains more than one field or if you set the multisource argument to True a multi-source uvfits file will be written. Because of limitations in the uvfits format you have to ensure that the data shape is fixed for all the data you intend to write to one fits file. See the general description of this tool for how you can select data to meet this condition.

The combinespw argument is used to control whether data from different spectral windows will be written as different entries in the fits FQ (frequency) table or combined as different IF’s within one entry in the FQ table. You should normally only set this to True if you know that the data from different spectral windows were observed simultaneously, and the data in the measurement set can be equally divided between all the spectral windows (i.e. each window should have the same width). Use of this switch is recommended for data to be processed in classic AIPS and difmap (if possible, e.g., standard dual IF observations).

The padwithflags argument is only relevant if combinespw is True. If true, it will fill in data that is ’missing’ with flags to fit the IF structure. This is appropriate if the MS had a few frequency-dependent flags applied, and was then time-averaged by split. If the spectral windows were observed at different times, padwithflags=True will add a large number of flags, making the output file significantly longer. It does not yet support spectral windows with different widths.

The fits GC (gain curve) and TY (system temperature) tables can be optionally written by setting the writesyscal argument to True. This is a rather WSRT-specific operation at the moment and may not work correctly for measurement sets from other telescopes.

The width argument is for channel averaging while outputing the data to the fits file. The default values of 1 will copy the channels of the input as is. The start channel number is the first channel of the spw expression. The number of output channels is determined by spw expression and the width. The width is the number of channels of the input data to make 1 channel of the output data.

One may overwrite the specified output file if it exists by specifying overwrite=True.

NOTE ON WEIGHTS

If the MS has no WEIGHT_SPECTRUM column, or if it does, but that column does not contain any data, ms.tofits() will compute the associated weight it writes to the uvfits file by taking the associated WEIGHT column value in the MS and dividing it by the number of channels associated with the spectral window of that visibility.

Arguments


Inputs
fitsfile	Name of the new uvfits file
	allowed:	string
	Default:
column	Data column to write, see above for options
	allowed:	string
	Default:	corrected
field	Field ids (0-based) or fieldnames to split out
	allowed:	any
	Default:	variant
spw	Spectral windows to split
	allowed:	any
	Default:	variant
width	DEPRECATED. Use mstransform to select and average data prior to running tofits().
	allowed:	int
	Default:	1
baseline	Antenna names or Antenna indices to select
	allowed:	any
	Default:	variant
time	Limit data selected to be within a given time range. Syntax is the defined in the msselection link
	allowed:	string
	Default:
scan	Limit data selected on scan numbers. Syntax is the defined in the msselection link
	allowed:	any
	Default:	variant
uvrange	Limit data selected on uv distance. Syntax is the defined in the msselection link
	allowed:	any
	Default:	variant
taql	For the TAQL experts, flexible data selection using the TAQL syntax
	allowed:	string
	Default:
writesyscal	Write GC and TY tables
	allowed:	bool
	Default:	false
multisource	Write in multisource format
	allowed:	bool
	Default:	false
combinespw	Export spectral windows as IFs
	allowed:	bool
	Default:	false
writestation	Write station name instead of antenna name
	allowed:	bool
	Default:	false
padwithflags	If combinespw==True, pad data with flags to fit IFs
	allowed:	bool
	Default:	false
overwrite	Overwrite output file if it exists?
	allowed:	bool
	Default:	false

Returns
bool

Example

      ms.open(’3C273XC1.MS’)
      ms.tofits(’3C273XC1.fits’, column=’DATA’);
      ms.done()

     This example writes  the observed data of a measurement set to a
     uvfits file.

      ms.open(’big.ms’)
      ms.tofits(’part.fits’, column=’CORRECTED’, field=[0,1], spw=[2]);
      ms.done()

     This example writes part (the first 2 fields and the third spectral
     window) of the corrected data to the fits file.

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