; FXPOL ;--------------------------------------------------------------- ;! Corrects VLBA polarization assignments ;# Task UV VLBI ;----------------------------------------------------------------------- ;; Copyright (C) 2000 ;; Associated Universities, Inc. Washington DC, USA. ;; ;; This program is free software; you can redistribute it and/or ;; modify it under the terms of the GNU General Public License as ;; published by the Free Software Foundation; either version 2 of ;; the License, or (at your option) any later version. ;; ;; This program is distributed in the hope that it will be useful, ;; but WITHOUT ANY WARRANTY; without even the implied warranty of ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the ;; GNU General Public License for more details. ;; ;; You should have received a copy of the GNU General Public ;; License along with this program; if not, write to the Free ;; Software Foundation, Inc., 675 Massachusetts Ave, Cambridge, ;; MA 02139, USA. ;; ;; Correspondence concerning AIPS should be addressed as follows: ;; Internet email: aipsmail@nrao.edu. ;; Postal address: AIPS Project Office ;; National Radio Astronomy Observatory ;; 520 Edgemont Road ;; Charlottesville, VA 22903-2475 USA ;----------------------------------------------------------------------- ;--------------------------------------------------------------- FXPOL LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC FXPOL Task to fix mislabelled VLBA polarizations and IFs INNAME Input UV file name (name) INCLASS Input UV file name (class) INSEQ 0.0 9999.0 Input UV file name (seq. no.) INDISK 0.0 9.0 Input UV file disk number OUTNAME Output UV file name (name) OUTCLASS Output UV file name (class) OUTSEQ 0.0 9999.0 Output UV file name (seq no.) OUTDISK 0.0 9.0 Output UV file disk number BANDPOL Polarization specification (see help) FQTOL Frequency tolerance for IF comparisons in kHz ---------------------------------------------------------------- FXPOL Task: The VLBA erroneously labels dual polarization experiments as having only one polarization and occasionally mislabels single polarization experiments. FXPOL allows the polarizations and IFs to be correctly labelled provided that no cross-polarization correlations are present. Adverbs: INNAME.....Input UV file name (name). Standard defaults. INCLASS....Input UV file name (class). Standard defaults. INSEQ......Input UV file name (seq. no.). 0 => highest. INDISK.....Disk no. 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. no.). 0 => highest. OUTDISK....Disk no. of output UV file. 0 => any. BANDPOL....A string specifying the polarization of each IF. The simplest form is a string of Rs and Ls. See HELP BANDPOL for the complete set of rules. ' ' => alternating RR and LL to the maximum number of IFs allowed. FQTOL......Frequency tolerance used when checking whether a pair input IFs should be assigned to the same output IF. Given in kHz. < 1.0 kHz => 10.0 kHz. ---------------------------------------------------------------- FXPOL: Corrects VLBA polarization assignments Documentor: Chris Flatters Related Programs: FITLD The VLBA correlator does not preserve polarization information unless it is operating in full polarization mode. This results in polarizations not being labelled correctly when both RR and LL polarizations are observed without RL and LR. Each VLBA correlator band is loaded into AIPS as a seperate IF and is assigned the same polarization. FXPOL takes a data set from the VLBA correlator and produces a new data set that has the correct IF and polarization assignments. Unfortunately, there is no reliable way to determine the polarization of each IF from the input data set and you must specify the polarization assigments using the BANDPOL adverb. Most VLBA setups assign odd-numbered bands to RCP and even- numbered bands to LCP. In this case BANDPOL should be set to '*(RL) ' (the default) and FXPOL will generate a new data set that is of equal size to the input data set but has two polarizations and half the number of IFs. This case normally applies if LISTR shows pairs of IFs with the same frequency and QHEADER shows one pixel on the STOKES axis with coordinate value RR but there may be exceptions to this rule when non-VLBA antennas are used. Most Mk3 and Mk4 VLBI setups reverse the polarizations and assign odd-numbered bands to LCP and even-numbered bands to RCP. In this case BANDPOL should be set to '*(LR) ' and the output data set will again be of equal size to the input data with two polarizations and half the number of IFs. This case normally applies if LISTR shows pairs of IFs with the same frequency and QHEADER shows one pixel on the STOKES axis with coordinate value LL but there may be exceptions to this rule when non-VLBA antennas are used. The vast majority of dual-polarization experiments fall into these two categories but more complicated setups are possible. For example, combined 50cm and 90cm observations will often assign 6 IFs to a single polarization in the 50cm band and 2 IFs to different polarizations in the 90cm band. FXPOL can deal with these cases but finding the correct setting for BANDPOL may be complicated because 1 - the VLBA correlator does not necessarily preserve the order of the bands (although each band-pair, 1-2, 3-4, etc., will be kept together), and 2 - FITLD will sort the bands into ascending order of frequency while preserving the relative ordering of bands that have the same reference frequency. In most cases you will be able to determine the appropriate setting for BANDPOL from your observing set-up but there may be cases where you will need to gain access to the records of the correlation to find out how the IFs in the AIPS file correspond to the bands in your original observing set-up. In general, the output files for the more complicated polarization configurations will be larger than the input files. In the worst possible case, the output file will be larger than the input file by a factor of 2. FXPOL will transfer the following tables to the output file, correcting them for the new IF/polarization structure as necessary. AN - antenna CL - calibration CQ - decorrelation information CT - CALC model FG - flags FQ - frequency GC - gain curves IM - interferometer model MC - CALC model components NX - index OB - orbital parameters PC - phase cal SU - sources TY - system temperature VT - tape statistics WX - weather All other tables will be deleted to avoid potential confusion. RESTRICTIONS 1 - FXPOL will not operate on files that already contain more than one polarization. 2 - FXPOL will not operate on files that have no IF axis nor on files that have no STOKES axis. 3 - FXPOL will not operate on files containing more than one frequency ID unless the same mapping from input IFs to output IFs can be applied to every frequency ID. 4 - FXPOL will do a limited amount of checking to ensure that no two output IFs have the same frequency and polarization. It will only check neighbouring IFs so FXPOL will not catch a case where, for example, three IFs at the same frequency are assigned RCP, LCP, RCP in that order. ----------------------------------------------------------------