; ACCOR ;--------------------------------------------------------------- ;! Corrects cross amplitudes using auto correlation measurements ;# Task UV VLBI CALIBRATION ;----------------------------------------------------------------------- ;; Copyright (C) 1995, 1996, 2001, 2010-2011 ;; 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 ;----------------------------------------------------------------------- ACCOR LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC ACCOR Correct cross correlations using auto correlations 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 # TIMERANG Timerange: 1-4 = start day,hr,min,sec 5-8 = end day,hr,min,sec SOLINT Solution interval (min) .GT.0 => no pay attention on scans .LT.0 => SOLINT=ABS(SOLINT) and average is terminated at the end of scans 0 => -2 min; see help ---------------------------------------------------------------- ACCOR Type: Task Use: ACCOR corrects amplitudes in cross-correlation spectra due to errors in sampler thresholds using measurements of auto-correlation spectra. Some correlators - JIVE apply this correction before writing out the data. For them, ACCOR should not be needed. On the other hand, ACCOR should also be benign since the auto-correlations should end up scaled to 1.0 and so cause essentially no correction to be made by ACCOR. For the VLBA correlators - both the older hardware correlator and the new software DiFX correlator - this correction is not done prior to reading the data into AIPS. For such data, ACCOR is required. 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. TIMERANG...Time range of the data to be used. In order: Start day, hour, min. sec, End day, hour, min. sec. Days relative to reference date. TIMERANGE = 0 => All available data are used SOLINT.....The solution interval (minutes). The mean values of spectra are averaged through the SOLUTION intervals following the start time in TIMERANGE. If SOLINT is more than the range of times in TIMERANG, the time of averaging is equal to the range of times in TIMERANG. If SOLINT > 0, the averaging does not pay attention to scan boundaries and several scans inside SOLINT may be averaged together. The averaging in this case may include more than one source ID or/and frequency ID. The source ID and frequency ID written to the output SN table is the same for the all rows of the SN table and equal 0. Use CALSOUR = '' when running CLCAL to convert this SN table to the relevant CL table. If SOLINT < 0, ABS(SOLINT) is used as the average interval and averaging is terminating at the end of the scan. The correct source IDs and frequency IDs appear in the output SN table for the each averaged interval. CALSOUR can be used in CLCAL to select calibrators in this case. Because of the gaps between scans, small intervals can appear at scan ends. Intervals less than 0.2*MIN(SOLINT, SVAN) are not recorded to the output SN table. 0 => -2 min ---------------------------------------------------------------- ACCOR: Task to correct amplitudes distorted by errors of samplers. Documentor: L.R. Kogan RELATED PROGRAMS: FITLD PURPOSE Calibration of a cross correlation spectrum supposes knowing parameters of samplers (digitizers in particular). Unfortunately these parameters are not identical to optimal ones and their real values are not known with enough accuracy. As a result the cross correlation spectrum amplitudes are measured with errors. These errors can be estimated having measured the mean value of auto correlation spectrum of a given interferometer's antennas. The theory of this correction is given by L. Kogan in VLBA SCIENTIFIC MEMO #9, 1995. The task estimates the mean values of the auto correlation spectra of all antennas of a given VLBI network and records them in a new SN table. The following application of the correction is provided by standard procedure of applying the SN table to an CL table (CLCAL) and following application this table with DOCALIB = 1 in relevant tasks. Some correlators apply this correction to the data before writing them out - notably the EVN JIVE correlator. The VLBA hardware and software (DiFX) correlators do NOT apply this correction to the data. Therefore, ACCOR is required for the VLBA correlators and any others that do not apply the correction. ACCOR should be benign (do nothing) for those correlators that do apply the correction prior to reading the data into AIPS. Note that you can always run ACCOR and look at the SN table produced with PRTAB or SNPLT to see if it was benign or not. COMMENTS This method of correction is especially effective in the case of TWO bits (FOUR levels) digitizers. Uncorrected amplitudes of cross correlation spectra can differ from real ones in this case up to 25%. The correction can minimize the errors to a bit of percent. The behavior of the samplers' errors is unknown. In many cases it is stable for hours for a given sampler. But cases of minute changing can not be excluded. The default value of SOLINT is 60 minutes. But smaller value 5-10 minutes can be recommended as well. The correction quality can be checked by POSSM (for example) with DOCALIB=1 and GAINUSE equaled to the CL table number gotten from ACCOR's SN table. The mean values of all auto correlation spectra would be close to unit. The task corrects other small errors of spectrum amplitudes caused by other errors besides samplers errors. The task requires TB order of data. If it is not the task UVSRT has to be applied to the data early.