Documentation on impol

Task: impol
Purpose: Compute polarized intensity and position angle from Q and U
Categories: 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.

Key: in
        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. 

Key: poli
        Up to two values; the output polarized intensity image and
        optionally, its associated error image (which will be constant).
        Default is no output images.

Key: polm
        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.

Key: pa
        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.

Key: sigma
        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

Key: sncut
        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

Key: pacut
        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.

Key: rm
        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

Key: options
        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

Key: device
        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.