[daip] Question to Polco

[Eric Greisen]

On 02/17/2016 06:33 AM, Rainer Beck wrote:
> Dear Eric,
>
> we are writing a paper about a new method to correct the polarization
> bias and want to compare with the method used in AIPS. Is it true that
> POLCO uses the Maximum Likelihood method from Killeen et al. (1986),
> while COMB with POLC uses something like Wardle & Kronberg (1974)?
>
> Best regards,
>
> Rainer.
>
The help file for POLCO explains its methods in some detail suggesting 
that at moderate and better S/N the Killeen method is used but that the 
Wardle/Kronberg method is used at loew S/N.

I reproduce the precursor comments from COMB's special bias routine:

       SUBROUTINE PDBIAS (P, RMS)
C-----------------------------------------------------------------------
C   Estimates the polarization bias in a polarization amplitude, P,
C   measured in the presence of Q and U RMS noise, RMS.  Returns the
C   corrected value.
C      The bias correction is such that the average bias is removed;
C   thus the average in the absence of a signal is zero.  Does table
C   look of values calculated by J. Condon in the range of P/RMS of
C   1.253 (the no signal limit) and 4 (the high SNR regime).  Does
C   second order Lagrange interpolation.  At lower values of P/RMS the
C   bias is a constant 1.253*RMS. Above a signal-to-noise ratio of 4,
C   use the formula:
C        normalized bias = 1 / (2 * s) + 1 / (8 * s**3),
C   where s is the true normalized flux density, iterating once to
C   estimate s from the normalized map flux density.  "Normalized" means
C   divided by the rms noise in the q and u maps.
C   Inputs:
C      P     R     On input, P is the observed total polarized intensity
C      RMS   R     The standard deviation of the (assumed equal)
C                  Gaussian distributions of the Stokes Q or U maps.
C   Output:
C      P     R     On output, P is the estimated intrinsic total
C                  polarized intensity.
C   Thanks to Jim Condon, Bill Cotton and the NVSS for this routine.
C-----------------------------------------------------------------------

I hope this helps - you may need to consult the NVSS references for the 
origin of this method.

Eric Greisen