AIPS HELP file for SWPOL in 31DEC21
As of Tue Oct 26 22:00:20 2021
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 SY, TY, CD 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 the normal/alternative IF switch is set to the wrong
position at one or more telescopes.
Calibration data in CL and SN tables may updated to
reflect the altered polarization streams at the discretion
of the user.
SWPOL will handle both linear and circular polarizations
but not a mixture of the two. SWPOL requires both both
parallel hands (RR and LL or XX and YY) to be present and
if any cross-hand (RL, LR, XY or YX) is present its
conjugate cross-hand musty 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) only. < 0 -> no table swap.
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),
(XX, YY), (RR, LL, RL, LR) or (XX, YY, XY, YX).
Some examples of the use of SWPOL to correct common errors are
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.
SWINIT: COMPLEX AXIS MUST BE AXIS ONE
This should not occur. If it does many other AIPS tasks will
also fail on your data.
SWINIT: MUST BE EITHER 2 OR 4 POLARIZATIONS PRESENT
SWINIT: POLARIZATIONS INCOMPLETE OR MIXED
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).