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calibrater.setsolvebandpoly - Function

	Package	synthesis
	Module	calibrater
	Tool	calibrater

Specialization of setsolve for polynomial B (bandpass) solving

Synopsis
setsolvebandpoly(table, append, degamp, degphase, visnorm, bpnorm, maskcenter, masksedge, refant, unset)

Arguments

table	in	Output calibration table name
		Allowed:	String
		Default:	' '
append	in	Append to existing table?
		Allowed:	Bool
		Default:	F
degamp	in	Polynomial degree for amplitude solution
		Allowed:	Int
		Default:	3
degphase	in	Polynomial degree for phase solution
		Allowed:	Int
		Default:	3
visnorm	in	Normalize data prior to solution
		Allowed:	Bool
		Default:	F
bpnorm	in	Normalize result?
		Allowed:	Bool
		Default:	T
maskcenter	in	Number of channels to avoid in center of each band
		Allowed:	Int
		Default:	1
masksedge	in	Fraction of channels to avoid at each band edge (in %)
		Allowed:	Int
		Default:	5
refant	in	Reference antenna
		Allowed:	Int
		Default:	-1
unset	in	If T, unset solve of this type
		Allowed:	Bool
		Default:	F

Returns
Bool

Description

This function is a specialization of the setsolve method which should be used to arrange for bandpass solving when polynomial solutions for B are desired, e.g., when SNR on calibrators is very low. Currently, this solving mode is available only for single polarization data.

Prior to the solution, the visibility data are averaged in time, and if multiple spectral windows are present (after running setdata), they are concatenated in frequency and one polynomial solution is delivered.

The solution is performed for both phase and amplitude, to the polynomial degrees specified in degamp and degphase (they can be different), and the result will be stored in the same table. The frequency domain of the solutions is limited to only the range of frequencies selected in setdata. When correcting data with these solutions (for other solves or with correct), only data within this domain will be corrected. Data outside (e.g., edge channels avoided in setdata for the solve), will not be corrected. Therefore, the same (or narrower) channel selection is recommended for all operations using solutions produced by this function and solve().

Like other solve types, the solve() function actually performs the solve according to the parameters set in this function, and for this type, it generates a plot file which shows the baseline spectra delivered to the solver (i.e., corrected according to any setapply() invocations, and time-averaged), with the derived solution (products of antenna-based bandpass polynomials) overlaid.

After solving, these solutions should be applied using type='BPOLY' in setapply.

The visnorm parameter is used to normalize the assembled spectral data, in a per baseline manner. If visnorm=T is used, this will have the effect of removing any non-frequency-dependent closure errors (e.g., as caused by source structure, or introduced by the instrument) from the data, and should be used with caution. The resulting solutions will be effectively normalized as well. When visnorm=F is used, closure errors in the data (as supplied to the solver) may be visible in the form of offsets between the data and solutions. For bandpass calibration, this is usually ok, as the shape of the bandpass is the most important aspect of the solution.

The bpnorm parameter controls normalization of the resulting solutions after the solve. Note that the baseline spectra plot does not show normalized solutions, even if bpnorm=T (the plot shows the data as provided to the core solver, and is generated before the solutions are normalized).

Note that use of bpnorm=T or visnorm=T will remove or prevent the storage of scaling information in the bandpass solutions. Sometimes it is desirable to store absolute scaling information in the bandpass solution; in these cases, bpnorm and visnorm must both be F.

The maskcenter and maskedge parameters control how many channels are ignored on-the-fly, at the center and edges of each input spectral window, respectively. To avoid edge channels, it is almost always better to flag these channels directly, or select against them in setdata. Aggressive use of maskedge (large values), will yield polynomial solutions which will tend to diverge at the edges (especially when the polynomial degree is also high), because maskedge does not change the frequency domain of the solutions. Such solutions should be used with caution in subsequent operations. (It is best to avoid use of maskedge.)

Example

c:=calibrater('ngc5921.ms');
c.setsolvebandpoly(table='bpoly',degamp=5,degphase=7);
c.solve();
c.done();

In this example, amplitude (degree 5) and phase (degree 7) Chebychev polynomial bandpasses are determined using the default parameters.