AIPS HELP file for SWPOL in 31DEC25
As of Wed May 21 21:34:12 2025
SWPOL: Task which swaps polarizations in a UV data base
INPUTS
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
HELP SECTION
SWPOL
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.
Adverbs:
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
EXPLAIN SECTION
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.
DIAGNOSTIC MESSAGES
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
Self-explanatory.
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).