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Section: User Commands (1)
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impol - Compute polarized intensity and position angle from Q and U  




image analysis  


IMPOL computes the total linearly polarized intensity (optionally debiasing it) and position angle images from Stokes Q and U images. Position angle is positive N -> E.  


Upto three values; the Q, U and I images, respectively. The I image is only needed if you want to compute the fractional polarization image as well or if you want to blank the output based upon an I S/N ratio. Wild card expansion is supported.
Up to two values; the output polarized intensity image and optionally, its associated error image (which will be constant). Default is no output images.
Up to values; the output fractional polarization image and optionally, its associated error image. You need to input an I image to keyword "in" for this.
Up to two values; the output position angle image and optionally, its associated error image (which will not be constant). These will be in degrees (but see OPTIONS=RADIANS), Default is no output images.
Up to 2 values; the mean standard deviation of the noise in the Q & U images (i.e. one number for them both), and the standard deviation of the I image.

These are required for debiasing (Q,U only), or for generating output error images, or for blanking the output. Try to make the Q,U value as accurate as possible for the debiasing. Perhaps measure it from a V image No default for sigma_QU, sigma_I defaults to sigma_QU

Up to 2 values. The first is the S/N ratio, P/SIGMA_QU, below which the output images are blanked (see also options=zero below). It is generally recommended that an SNCUT of at least 2 is used. The second value, which is only valid when you have input an I image and sigma, is the S/N ratio, I/SIGMA_I, below which output images are blanked (defaults to no I based blanking) The default is 2.0 and 0.0
The output images are blanked if the error in the position angle image (degrees or radians depending on OPTIONS) is greater than this value. This is active even if you don't output the PA image. Note that there is no equivalent for the output error of the POLI image because the error is constant and equal to SIGMA. Keyword SNCUT essentially takes care of this. The default is no position angle error based blanking.
After computing the position angle image, rotate the position angles back to zero wavelength by an amount specified by RM (rad/m**2). Better to use IMRM to generate the rotation measure and zero wavelength position angle images. Default is 0.0
Task enrichment options. Minimum match is active.

"bias" If computing polarized intensity, do NOT remove the Ricean

           bias in the image.  By default, the bias is removed to first
           order with P = sqrt(P_obs**2 - sigma**2)   You should have
           a very good reason for using this option (e.g. a detection
           experiment).  See VLA memo no. 161 by Patrick Leahy for
           more details of bias removal.

"zero" When the output pixel is clipped because the debiasing

           fails (P**2 may become negative), setting OPTIONS=ZERO
           will cause the output polarized intensity image (not the
           position angle image) pixel to be set to 0.0 rather than
           being masked out.   This is very important if you are
           interested in doing statistics on a region of low
           polarized intensity S/N ratio.  If you leave the region
           masked rather than zeroed, you will bias the statistics
           in that region -- zero is a better estimate of the pixel
           than just excluding it from the statistics (provided the
           clip level is sufficiently small). Residual bias in the
           statistical results from the area then depend upon how
           well the bias remover works and at what level clipping
           was performed.  See VLA memo no. 161 by Patrick Leahy.

"radians" Output the position angle image in radians instead

           of degrees.

"relax" Only warn about image axis descriptor mismatches

           instead of giving a fatal error
PGPLOT device on which to draw a plot showing the effect of bias in polarized intensity images. It plots true polarized intensity versus the bias, which is the estimated polarized intensity minus the true polarized intensity. Three estimators are shown; observed, first order, and maximum likelhood. It is assumed that sigma_P = 1 in these plots. Because these plots are drawn following a monte carlo simulation of some 15,000 trials of the noise, you will need to be patient. You can just make this bias plot without actually working on any data if you wish. See also VLA memo no. 161 by Patrick Leahy. Default is no plot.




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Time: 18:35:38 GMT, July 05, 2011