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calibrater.posangcal - Function

	Package	synthesis
	Module	calibrater
	Tool	calibrater

Apply position angle calibration to an existing cal table

Synopsis
posangcal(tablein, tableout, posangcor)

Arguments

tablein		Input calibration table name
		Allowed:	String
tableout		Output calibration table name. Default is input table
		Allowed:	String
posangcor		Position angle corrections (degrees)
		Allowed:	Vector of Floats
		Default:	[]

Returns
Bool

Description

This function is used to apply position angle calibration for observations made using circularly polarized feeds. According to the Measurement Equation formalism, this correction should be applied to a D (instrumental polarization) calibration table.

If no D calibration is performed (and thus no such table is available), the correction can be applied to a G table, but it should NEVER be applied to both, and always applied to a D table if one is available. An input table must be specified. If no output table is specified, then the input table will be modified in place.

Specify, as a vector of values, a position angle adjustment (in degrees) for each spectral window. If only one value is specified, it will be duplicated to all spectral windows; otherwise, the number of values specified must match the number of spectral windows. The sign convention for the position angle adjustment is such that the specified value is the that which, when added to the position angle implied by the data, will yield the correct position angle. For example, if G-, D-, and P-calibrated data for 3c286 suggests a position angle of 45 degrees, the posangcor value should be -12 degrees as this will yield the correct position angle of 33 degrees when added. In general, posangcor equals correct position angle minus observed position angle.

A future version of this function will have an option to recognize standard position angle calibrators and determine the correction automatically.

Example

c:=calibrater('3C273XC1.MS');
c.posangcal(tablein='3C286.dcal', tableout='3C286.dpacal', 
             posangcor=[-12.0, 54.0]);
c.done();

This example takes an existing calibration table containing D Jones matrices, and applies a position angle calibration of 45 and 54 degrees to spectral windows 1 & 2, respectively, writing the result to a new table. The observed position angles for 3C286 must have been 45 and -21 degrees; the corrections specified yield the correct value of 33 degrees when added to the observed values.