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NRAO Home > CASA > CASA Task Reference Manual |
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0.1.91 rmfit
Requires:
Synopsis
Calculate rotation measure.
Arguments
Inputs |
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imagename |
| Name(s) of the input image(s). Must be specified.
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| allowed: | any |
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| Default: | variant
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rm |
| Output rotation measure image name. If not specified,
no image is written.
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| allowed: | string |
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| Default: |
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rmerr |
| Output rotation measure error image name. If not
specified, no image is written.
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| allowed: | string |
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| Default: |
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pa0 |
| Output position angle (degrees) at zero wavelength
image name. If not specified, no image is written.
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| allowed: | string |
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| Default: |
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pa0err |
| Output position angle (degrees) at zero wavelength error
image name. If not specified, no image is written.
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| allowed: | string |
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| Default: |
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nturns |
| Output number of turns image name. If not specified,
no image is written.
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| allowed: | string |
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| Default: |
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chisq |
| Output reduced chi squared image name. If not specified,
no image is written.
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| allowed: | string |
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| Default: |
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sigma |
| Estimate of the thermal noise. A value less than 0 means
auto estimate.
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| allowed: | double |
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| Default: | -1 |
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rmfg |
| Foreground rotation measure in rad/m/m to subtract.
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| allowed: | double |
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| Default: | 0.0 |
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rmmax |
| Maximum rotation measure in rad/m/m for which to
solve. IMPORTANT TO SPECIFY.
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| allowed: | double |
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| Default: | 0.0 |
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maxpaerr |
| Maximum input position angle error in degrees to allow
in solution determination.
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| allowed: | double |
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| Default: | 1e30 |
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bool
Example
PARAMETER SUMMARY
imagename Name(s) of the input image(s).
rm Output rotation measure image name. If not specified, no image is written.
rmerr Output rotation measure error image name. If not specified, no image is written.
pa0 Output position angle (degrees) at zero wavelength image name. If not specified, no image is written.
pa0err Output position angle (degrees) at zero wavelength error image name. If not specified, no image is written.
nturns Output number of turns image name. If not specified, no image is written.
chisq Output reduced chi squared image name. If not specified, no image is written.
sigma Estimate of the thermal noise. A value less than 0 means auto estimate.
rmfg Foreground rotation measure in rad/m/m to subtract.
rmmax Maximum rotation measure in rad/m/m for which to solve. IMPORTANT TO SPECIFY.
This task generates the rotation measure image from stokes Q and U measurements at several
different frequencies. You are required to specify the name of at least one image with a polarization
axis containing stokes Q and U planes and with a frequency axis containing more than two pixels. The
frequencies do not have to be equally spaced (ie the frequency coordinate can be a tabular coordinate).
It will work out the position angle images for you. You may also specify multiple image names, in which
case these images will first be concatenated along the spectral axis using ia.imageconcat(). The requirments
are that for all images, the axis order must be the same and the number of pixels along each axis must
be identical, except for the spectral axis which may differ in length between images. The spectral axis need
not be contiguous from one image to another.
See also the fourierrotationmeasure
function for a new Fourier-based approach.
Rotation measure algorithms that work robustly are few. The main
problem is in trying to account for the $n- \pi$ ambiguity (see Leahy et
al, Astronomy \& Astrophysics, 156, 234 or Killeen et al;
http://www.atnf.csiro.au/\verb+~+nkilleen/rm.ps).
The algorithm that this task uses is that of Leahy et al. in see
Appendix A.1. But as in all these algorithms, the basic process is that
for each spatial pixel, the position angle vs frequency data is fit to
determine the rotation measure and the position angle at zero wavelength
(and associated errors). An image containing the number of $n- \pi$ turns
that were added to the data at each spatial pixel and for which the best fit
was found can be written. The reduced chi-squared image for the fits can
also be written.
Note that no assessment of curvature (i.e. deviation
from the simple linear position angle - $\lambda^2$ functional form)
is made.
Any combination of output images can be written.
The parameter sigma gives the thermal noise in Stokes Q and U.
By default it is determined automatically using the image data. But if it
proves to be inaccurate (maybe not many signal-free pixels), it may be
specified. This is used for calculating the error in the position angles (via
propagation of Gaussian errors).
The argument maxpaerr specifies the maximum allowable error in
the position angle that is acceptable. The default is an infinite
value. From the standard propagation of errors, the error in the
linearly polarized position angle is determined from the Stokes Q and
U images (at each directional pixel for each frequency). If the position angle
error for any pixel exceeds the specified value, the position angle at that pixel
is omitted from the fit. The process generates an error for the
fit and this is used to compute the errors in the output
images.
Note that maxpaerr is not used to mask pixels in the output images.
The argument rmfg is used to specify a foreground RM value. For
example, you may know the mean RM in some direction out of the Galaxy,
then including this can improve the algorithm by reducing ambiguity.
The parameter rmmax specifies the maximum absolute RM value that
should be solved for. This quite an important parameter. If you leave
it at the default, zero, no ambiguity handling will be
used. So some apriori information should be supplied; this
is the basic problem with rotation measure algorithms.
EXAMPLES
# Calculate the rotation measure for a single polarization image
rmfit(imagename="mypol.im", rm="myrm.im", rmmax=50.0)
# calculate the rotation measure using a set of polarization images from
# different spectral windows or bands.
rmfit(imagename=["pol1.im", "pol2.im", "pol3.im", rm="myrm2.im", rmmax=50.0)
More information about CASA may be found at the
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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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