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imagepol.fourierrotationmeasure - Function

	Package	general
	Module	images
	Tool	imagepol

Find Rotation Measure (Fourier approach)

Synopsis
fourierrotationmeasure(complex, amp, pa, real, imag, zerolag0)

Arguments

complex	in	Output complex linear polarization image file name
		Allowed:	String
		Default:	Unset
amp	in	Output linear polarization amplitude image file name
		Allowed:	String
		Default:	Unset
pa	in	Output linear polarization position angle (degrees) image file name
		Allowed:	String
		Default:	Unset
real	in	Output linear polarization real image file name
		Allowed:	String
		Default:	Unset
imag	in	Output linear polarization imaginary angle image file name
		Allowed:	String
		Default:	Unset
zerolag0	in	Force zero lag to 0 ?
		Allowed:	T or F
		Default:	F

Returns
T or fail

Description

This function (short-hand name frm) will only work if you constructed the Imagepol tool from an image containing Stokes Q and U, and a regular frequency axis. It Fourier transforms the complex linear polarization (Q+iU) over the spectral axis to the rotation measure axis. Thus, if your input image contained RA/DEC/Stokes/Frequency, the output image would be RA/DEC/RotationMeasure. The Rotation Measure axis has as many pixels as the spectral axis.

This method enables you to see the polarization as a function of Rotation Meausure. Its main use is when searching for large RMs. See Killeen, Fluke, Zhao and Ekers (1999, preprint) for a description of this method (or http://www.atnf.csiro.au/~nkilleen/rm.ps) and its advantages over the traditional method (rotationmeasure) of extracting the Rotation Measure.

Although you can write out the complex polarization image with the argument complex, you can't do much with it because Image tools cannot yet be constructed from complex images. Hence you can also write out the complex linear polarization image in any or all of the other forms.

The argument zerolag0 allows you to force the zero lag (or central bin) of the Rotation Measure spectrum to zero (effectively by subtracting the mean of Q and U from the Q and U images). This may avoid Gibbs phenomena from any strong low Rotation Measure signal which would normally fall into the central bin.

Example

- p := imagepoltestimage(outfile='iquv.im', rm=[5.0e5,1e6], nx=8, ny=8, nf=512, 
                         f0=1.4e9, bw=8e6)
- p.frm(amp='amp')
- amp := image('amp')
- amp.view()                     # And reorder to put RM along X-axis

In this example we give two RM components and recover them.