AIPS HELP file for POLCO in 31DEC26

As of Sat May 9 20:07:31 2026

INPUTS

INNAME                             Input image name (name)
INCLASS                            Input image name (class)
INSEQ           0.0      9999.0    Input image name (seq. #)
INDISK                             Input image disk drive #
OUTNAME                            Output image name (name)
OUTCLASS                           Output image name (class)
OUTSEQ          0.0      9999.0    Output image name (seq. #)
OUTDISK                            Output image disk drive #
PIXSTD          0.0      9999.0    Sigma of noise on Q or U map
PCUT            0.0                Blank below PCUT sigma.
                                   <= 0 => no blanking
OPCODE                             'OLD' use new method, 'ZERO'
                                   and 'ZOLD' blank with zeros,
                                   else use magic blanking
BADDISK                            Disks to avoid for scratch

HELP SECTION

POLCO
Type: Task
Use:  POLCO is a task to correct total polarized intensity (P) images
      for the bias introduced when combining the Stokes Q and U images
      which are assumed to have a noise distribution obeying Gaussian
      statistics with a standard deviation sigma.  The P image obeys the
      Rice distribution, and at high signal-to-noise, Gaussian
      statistics are approached (see Vinokur, Ann. d'Ap., 1965, 28,
      412).  One needs to estimate the intrinsic polarized intensity
      (Pint) given an observed polarized intensity (Pobs).   If Pint=0,
      then the bias in the observed image is Pobs(Pint=0) = sqrt(pi/2) *
      sigma.  This task uses the maximum likelihood (ml) estimator of
      Pint from Pobs (see Killeen, Bicknell, and Ekers, 1986, ApJ, 302,
      306). The ml estimator does not provide an analytic solution.
      Therefore,  this task uses an asymptotic expression together with
      an empirically determined correction.   There is no significant
      loss of accuracy with this procedure.   The magnitude of the
      correction is approximately Pint/Pobs ~ 0.8 at Pobs/sigma=2 and
      Pint/Pobs ~ 0.9 at Pobs/sigma=2.5.  Pint/Pobs asymptotes to unity
      with increasing Pobs/sigma.  The user must input sigma (Jy).

      The parameter PCUT is the ratio Pobs/sigma below which the output
      image will be blanked.  This may be used to avoid the (mis)use of
      low signal-to-noise polarization data.  Furthermore, it is likely
      that a more conservative value of PCUT (e.g. 3-sigma) may be more
      desirable.  On the other hand, blanking will give a false sense of
      the noise on the image (underestimating by about a factor of 2).
      An empirical method for handling lower values of Pobs/sigma,
      attributed to Wardle and Kronberg, is used for Pobs/sigma < 2.
      This method gives negatives for the lowest values and represents
      the noise much more correctly.  Pobs <= 0 (which should not occur)
      are still blanked.

      In 31DEC26, a new task was written to study this problem.  The
      AIPS random number generator was used to generate 1000000 random
      numbers with rms 1.0.  Then, for 1000000 evaluations of Q and U
      using these random numbers, the mean and median polarization are
      computed with true values of Q and U from 0 to 50 by 0.05 are
      computed.  The difference between the average of the mean and
      median and the correct value is determined and then compared to
      the algorithms in COMB and POLCO.  A text file suitable for AIPS
      task PLOTR is produced.  This provided three functional forms and
      a lookup table to cover the full range of polarizations.  This
      function may be used by specifying any OPCODE except 'OLD' or
      'zold' if zeros are desired as the blanking value.  This task is
      called POLTS.

      The task COMB with OPCODE 'POLC' performs the same operation with
      a somewhat different algorithm for the correction.  COMB has the
      option of having an image of the sigmas rather than a single
      value.  POLTS suggests that the algorithm in COMB and the older
      algorithm in POLCO produce nearly identical results and slightly
      under correct the polarization even at S/N of 25.  The new
      algorithm appears to be marginally better.

Adverbs:
  INNAME......First image name (name).  blank=>any
  INCLASS.....First image name (class).  blank=>any
  INSEQ.......First image name (seq. #).  0=>any
  INDISK......Disk drive # for the first image.  0=>any
  OUTNAME.....Output image name (name)
  OUTCLASS....Output image name (class)
  OUTSEQ......Output image name (seq. #)
  OUTDISK.....Output image disk drive #
  PIXSTD......Input sigma of noise distribution on Q or U image (Jy)
  PCUT........Blank below (Polarized flux/sigma)=PCUT
  OPCODE......Choice of algorithm and type of blanking.
              'OLD'  use older algorithm with magic blanking
              'ZOLD' use older algorithm with zero blanking
              'ZERO' use new algorithm with zero blanking
              other  use new algorithm with magic blankin
  BADDISK.....Disks to avoid for scratch

EXPLAIN SECTION