As of Sat Apr 20 14:13:14 2024

SWPOL: Task which swaps polarizations in a UV data base


INNAME                             Input UV file name (name)
INCLASS                            Input UV file name (class)
INSEQ             0.0     9999.0   Input UV file name (seq. #)
INDISK            0.0        9.0   Input UV file disk unit #
OUTNAME                            Output UV file name (name)
OUTCLASS                           Output UV file name (class)
OUTSEQ           -1.0     9999.0   Output UV file name (seq. #)
OUTDISK           0.0        9.0   Output UV file disk unit #.
ANTENNAS                           Antennas to swap
SUBARRAY          0.0       99.0   Selected subarray
DOCALIB                            Swap calibration info?
DOTABLE          -1.0        1.0   >= 0 > swap fg, SY, TY, CD
                                          and more tables
FQCENTER                           >= 0 -> center frequency axis


Task:  This task swaps polarization data streams in a UV data
       file. This allows data sets in which the polarization
       streams have been mislabelled to be corrected.
       Polarization streams may be mislabelled in VLA data if
       the transfer switch is set incorrectly or in VLBI data if
       the normal/alternative IF switch is set to the wrong
       position at one or more telescopes.  They may also be
       mislabeled if the "X" polarization is horizontal rather
       than the expected vertical.

       Calibration data in CL, SN, BP, and PD tables may updated
       to reflect the altered polarization streams at the
       discretion of the user.  SY, TY, CD, AN, GC, IM, MC, and OF
       tables may also be swapped if desired.  FG tables will be
       swapped on DOTABLE >= 0, but certain combinations of
       flagged antennas/polarizations may not work well with the
       ANTENNAS adverb.

       SWPOL will handle both linear and circular polarizations
       but not a mixture of the two. SWPOL requires both parallel
       hands (RR and LL or VV and HH) to be present and if any
       cross-hand (RL, LR, VH or HV) is present its conjugate
       cross-hand must also be present.

  INNAME.....Input UV file name (name).      Standard defaults.
  INCLASS....Input UV file name (class).     Standard defaults.
  INSEQ......Input UV file name (seq. #).    0 => highest.
  INDISK.....Disk drive # of input UV file.  0 => any.
  OUTNAME....Output UV file name (name).     Standard defaults.
  OUTCLASS...Output UV file name (class).    Standard defaults.
  OUTSEQ.....Output UV file name (seq. #).   0 => highest unique
  OUTDISK....Disk drive # of output UV file. 0 => highest with
             space for the file.
  ANTENNAS...Antennas for which polarizations are to be swapped.  If
             all entries are zero then polarizations are swapped for
             the whole array. If any entry is negative then all
             antennae *except* those listed are swapped.
  SUBARRAY...Subarray to process.            0 => subarray 1
  DOCALIB....If > 0 then swap calibration information for selected
             antennas in CL and SN tables
  DOTABLE....>= 0 -> swap polarizations in monitor tables (SY, TY, and
             CD) plus FG tables, AN, GC, IM, MC, and OF tables and,
             when ANTENNAS=0, PP, BD, and,BL tables.
             < 0 -> do not swap these.
  FQCENTER...>= 0 => Change frequency axis reference pixel to
                     Nchan / 2 + 1
             else => do not change reference pixel


SWPOL: Task to swap polarizations on one or more antennae
Documentor: C. Flatters, NRAO-AOC (preliminary version)
Related tasks: Most other UV tasks.

SWPOL is intended to correct UV data sets in which the
polarization data streams have been mislabelled at one or more
telescopes. It handles either linear (X and Y are swapped for
designated antennae) or circular polarization (R and L are
swapped for designated antennae). In order to allow
polarizations to be swapped for arbitrary subsets of telescopes
there are a number of restrictions on the form of the input
data. Principally, the polarization combinations in the data
must correspond to one of the following sets: (RR, LL),
(VV, HH), (RR, LL, RL, LR) or (VV, HH, VH, HV).

Some examples of the use of SWPOL to correct common errors are
given below.

Example 1: The transfer switch is set incorrectly at the VLA.
           This leads to the labels for right and left circular
           polarization being applied to the wrong channel for
           the whole array. In this case set up the input and
           output file specifications and set the ANTENNAS array
           to all zeros.

Example 2: One telescope in a VLBI experiment has set the
           normal/alt switch to the wrong position. This leads
           to RCP and LCP being recorded on the wrong tracks at
           that station; the diagnostic is wildly different
           amplitudes (commonly differing by a factor of 10) in
           the two parallel hands on baselines including that
           antenna. The solution is to find the number of the
           bad antenna using the 'MATX' output of LISTR, or using
           PRTAN if you already know the name of the telescope,
           and to set antennas using 'ANTENNAS = t, 0', where t
           is the antenna number of the bad telescope. DOCALIB
           should be set to true (> 0) to preserve phase-cals.


The following messages indicate that the input data is not in a
form that SWPOL can handle.

    This should not occur. If it does many other AIPS tasks will
    also fail on your data.


    The data contains a mixture of linear and circular
    polarizations or has a fragmentary set of polarizations (for
    example one parallel hand and one cross hand).