; BLAVG ;--------------------------------------------------------------- ;! Average cross-polarized UV data over baselines. ;# TASK UV Calibration ;----------------------------------------------------------------------- ;; Copyright (C) 1996, 2000, 2004, 2006 ;; 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 ;----------------------------------------------------------------------- ;--------------------------------------------------------------- BLAVG LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC BLAVG Task to baseline average cross-polarized UV data. 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 # SOURCES Source list. See important note in HELP file! TIMERANG Time range to use. FREQID Freq. ID to select. For phase alignment: BCHAN 0.0 2048.0 Start channel 0=>all ECHAN 0.0 2048.0 End channel 0=>all BIF -100.0 100.0 Lowest IF number 0=>all EIF 0.0 100.0 Highest IF number 0=>all ANTENNAS Antennas to use FLAGVER Flag table version SUBARRAY 0.0 1000.0 Sub-array, 0=>all DOCALIB -1.0 101.0 > 0 calibrate data & weights > 99 do NOT calibrate weights GAINUSE Cal table to apply 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 # SOLINT 0.0 Pre-averaging time (min) REFANT 0.0 100.0 Reference antenna ---------------------------------------------------------------- BLAVG Task: This task averages cross-polarized UV data over baselines to allow R-L phase and delay offsets between IFs to be determined by FRING and POLSN. Adverbs: INNAME.....Input UV file name (name). Sort must be 'TB'. 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. SOURCES....List of sources to process. '*' = all; a "-" before a source name means all except ANY source named. NOTE! Selection of only ONE source is not implemented correctly -> use TIMERANG instead. TIMERANG...Time range of the data to average. In order: Start day, hour, min. sec, end day, hour, min. sec. Days relative to ref. date. FREQID.....Frequency identifier to select (you may determine which is applicable from the OPTYPE='SCAN' listing produced by LISTR). BCHAN......First channel to use for phasing. 0=>1 ECHAN......Highest channel to use. 0=>all higher than BCHAN BIF........First IF to use for phasing. 0=>all. <0 => don't use this IF. EIF........Highest IF to use. 0=>all higher than BIF. Ignored, if BIF<0. ANTENNAS...A list of the antennas to include in the average. If any number is negative then all antennas listed are NOT to be used and all others are. All 0 => use all. FLAGVER....Specifies the version of the flagging table to be applied. 0 => highest numbered table. <0 => no flagging to be applied. SUBARRAY...Sub-array number to use. 0=>all; if DOCAL > 0 then multiple subarrays (SUBARRAY = 0) are not allowed. DOCALIB....If true (>0), calibrate the data using information in the specified Cal (CL) table for multi-source or SN table for single-source data. Also calibrate the weights unless DOCALIB > 99 (use this for old non-physical weights). GAINUSE....version number of the CL or SN table to apply to the data. 0 => highest. OUTNAME....Output UV file name (name). Standard defaults. OUTCLASS...Output UV file name (class). Standard defaults. OUTSEQ.....Output UV file name (seq. #). Standard defaults. OUTDISK....Disk drive # of output UV file. 0 => highest with space for the file. SOLINT.....Pre-averaging interval (min). 0 => 1.0 REFANT.....Reference antenna (must be given). --------------------------------------------------------------- BLAVG: Task to average cross-polarized UV data over baselines DOCUMENTOR: K. J. Leppanen NRAO/SOC RELATED PROGRAMS: FRING, POLSN, CRSFRING (procedure) PURPOSE BLAVG averages cross-polarized data over baselines and time. The purpose is to enable all available data to be used for deriving R-L phase and delay differences between IFs. These differences must be found and removed in order to average cross-hand data over frequency channels and IFs. USAGE BLAVG should be used only after parallel hand fringe fitting has been completed. It is essential that all the RCP and LCP fringe fit solutions have been properly referenced to a single reference antenna using SNSMO (not by FRING; i.e. use DPARM(7)=1 in FRING). The success of this referencing can be checked by plotting the R-L difference of the phase solutions for all antennas using SNPLT, STOKES='DIFF'. The differences should be zero or constant within a few degrees, depending whether the constant IF differences were removed before the fringe fitting or not. Running BLAVG on the fringed data produces a new UV file containing only cross-polarized data on one baseline (average over the baselines specified by the input adverbs). Run FRING on it using the SAME reference antenna, and set DOCALIB and ANTENNAS to 0. It is also important to set the delay and rate windows to their default values, and the input averaging interval DPARM(4) (given in seconds) to match the pre-averaging interval used by BLAVG. See comment on SOLINT below. Process the new SN table with POLSN and apply it to the original CL table (the one used by BLAVG) bu using CLCAL. At this point the phase and SB delay differences between IF's of R and L systems should have been removed, which may be checked by plotting cross-hand data with POSSM. PRINCIPLE OF OPERATION BLAVG is based on the fact that after successful parallel hand fringe fit the cross-hand visibilities on all baselines show the same phase and delay offsets between the IFs (RL and LR offsets show opposite signs). Thus averaging over baselines can be used to reduce noise in the typically weak cross-hand data. Due to the unknown phase offsets between baselines (caused by source polarization structure and station gain errors), the phases must be aligned before averaging visibilities from different baselines. The averaging is done in two steps: first visibility records on a single baseline are pre-averaged over the period specified by SOLINT, and then the averaged visibilities from different baselines are added together. Phasing is only applied in this second step, and is done by complex channel-by-channel division as the accumulation progresses. The IFs and channels used for the phasing can be controlled by the input adverbs, the default is to use all. However, the average itself always uses all IFs and channels. Finally the phases of the finished averages are rotated to force the phase of the reference IF (usually 1) to zero. Note that this method of averaging will retain ONLY the relative phase and delay differences between the IFs. The BLAVG output will only have two polarizations, RL and LR, marked as RR and LL, for FRING (see comment for REFANT). COMMENTS BIF,EIF: If data are bad or missing at some IFs and antennas in the specified time range, these should be avoided using adverbs BIF and EIF, or ANTEN. SOLINT: Should be set roughly equal to the coherence time since this gives the best preaverage SNR on a given baseline before aligning the phases and averaging different baselines. When running FRING, SOLINT should be set to the anticipated phase stability time scale of the station electronics. For example, if the phase differences between IFs (as obtained from parallel hand fringe fit) show only negligible variation over the period of observation, one can use a solution interval covering the whole observation. Coherent use of all the data is possible because of reference IF phase normalization. REFANT: Use the same reference antenna used for parallel hand fringe fit. Also, use this reference antenna for the subsequent cross-hand fringe fit of the output. REFANT affects numbering of the output baseline and labeling of the polarizations. If REFANT is the first antenna of the output baseline RL->LL & LR->RR; if is it the second RL->RR & LR->LL. BLAVG works only with TB sorted data for now.