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NRAO Home > CASA > CASA Toolkit Reference Manual |
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imagepol.fourierrotationmeasure - Function
1.1.4 Find Rotation Measure (Fourier approach)
Description
This function will only work if you attach the Imagepol tool (using open) to
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/$\sim$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 handle 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.
Arguments
Inputs |
| ||
complex |
| Output complex linear polarization image file name.
Default is unset. | |
| allowed: | string |
|
| Default: |
| |
amp |
| Output linear polarization amplitude image file name
Default is unset. | |
| allowed: | string |
|
| Default: |
|
|
pa |
| Output linear polarization position angle (degrees)
image file name Default is unset.
| |
| allowed: | string |
|
| Default: |
|
|
real |
| Output linear polarization real image file name Default
is unset.
| |
| allowed: | string |
|
| Default: |
|
|
imag |
| Output linear polarization imaginary angle image file
name Default is unset.
| |
| allowed: | string |
|
| Default: |
|
|
zerolag0 |
| Force zero lag to 0 ?
| |
| allowed: | bool |
|
| Default: | false |
|
bool
Example
"""
#
print "\t----\t fourierrotationmeasure Ex 1 \t----"
po.imagepoltestimage(outfile=’iquv.im’, rm=[5.0e5,1e6], nx=8, ny=8, nf=512,
f0=1.4e9, bw=8e6)
po.fourierrotationmeasure(amp=’amp’)
ia.open(’amp’)
ia.statistics()
#viewer() # And reorder to put RM along X-axis
#
"""
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Copyright © 2016 Associated Universities Inc., Washington, D.C.
This code is available under the terms of the GNU General Public Lincense
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