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Continuum polarimetry case

GUI Interface:
We want to form a linear polarization intensity image, e.g. P = $\sqrt{{Q^2 + U^2}}$ and a vector image of the electric vectors of the polarization, e.g. (Q + iU) as described in the Image analysis chapters of Getting Results. First, start an image polarization tool which provides access to a slew of tools that are useful for analyzing polarization images:

impolC:=imagepol(infile="0957+561.im2");   # Start the image polarization tool 
                                           #  for the image 0957+561.im2

Create a linear polarization intensity tool (leave all inputs to their default values):

linpolintC:=impolC.linpolint();            # Form linear polarization intensity
                                           #  image tool, linpolint
linpolintC.subim(outfile='0957+561.pint'); # Form the subim tool and write 
                                           #  the image 0957+561.pint
                                           #  to a file on disk
subim.done();                              # Close the tool (no longer needed)
linpolintC.done();                         # Close the tool (no longer needed)
impolC.complexlinpol(outfile="0957+561.cmplx");
                                           # Create a vector image of the
                                           # electric vectors of the polarization
impolC.done();                             # Close the tool (no longer needed)

You should now have two image files called 0957+561.pint & 0957+561.cmplx on disk as well as the original 0957+561.im2 polarization image. Follow the instructions above to load the images into the viewer:

• Load 0957+561.im2 as a raster image
  
• Load 0957+561.pint as a contour image
  
• Load 0957+561.cmplx as a vector map

Bring up the Image Analysis GUI and determine the RMS of the polarization intensity image using the 'Statistics' panel (for the 0957+561.pint image, the RMS should be $\sim$ 11 mJy beam^-1). We will use this value to set a mask expression in the 0957+561.cmplx vector image to get rid of polarization vectors that are below the RMS level.

Left-click on the 'Adjust' button to bring up 3 Adjustment panels, one for each image. Then:

• Adjust the color map for 0957+561.im2 (Color palette of choice: Greyscale 1 for black & white on the Viewer, Greyscale 2 for output to a psfile).
• Adjust the size of the polarization vectors in 0957+561.cmplx using the 'amplitude scale factor', 'x-increment', and 'y-increment'.
• Set the mask expression in the 0957+561.cmplx image to be: '0957+561.pint' > 0.00033 (3$\sigma$ in the polarization intensity image). Note, the name 0957+561.pint must be in quotes because it contains a + sign which will be interpreted as an expression.

Now, calibrate the electric-vector positions angle (EVPA):
EVPA  = 0.5 tan^-1(U/Q) = 0.5*(R-L phase difference).
First, it is necessary to specify the true ratio U/Q found for the polarization calibrator, 3C 286. During the polarization calibration, we found for 3C 286 ( I = 7.462  Q = + 0.4982  U = - 0.6988  V = 0), thus, the measured R-L phase difference is: tan^-1(- 0.6988/0.4982) = - 54.5^o. Since U is negative and Q is positive, this is in the 4^th quadrant.

From the VLA calibration web page (http://www.aoc.nrao.edu/$\sim$gtaylor/calman/polcal.html) we find the true R-L phase difference for 3C 286 is 66^o. Thus, our phases should be rotated by 66 + 54.5 = 120.5^o or EVPA = 0.5*120.5^o. In the 'Adjust' panel for the 0957+561.cmplx image, enter Extra rotation = 120.5/2. To plot the magnetic field vectors, enter Extra rotation = 120.5/2 + 90. The final images are shown in Fig. 1.24.