; FRCAL ;--------------------------------------------------------------- ;! Faraday rotation self calibration task ;# Task AP OOP calibration ;----------------------------------------------------------------------- ;; Copyright (C) 1995, 2008 ;; 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 ;----------------------------------------------------------------------- FRCAL LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC FRCAL: Faraday rotation self calibration task INNAME Input UV data (name) INCLASS Input UV data (class) INSEQ Input UV data (seq. #) INDISK Input UV data disk drive # IN2NAME Model Image file name IN2CLASS Model Image file class IN2SEQ Model Image file seq IN2DISK Model Image file disk IN2VER Model Image CC version number NITER Number of CC comps. to use. PMODEL Source polarization model. OUTNAME Output uvdata name (name) OUTCLASS Output uv data class. OUTDISK Output uvdata disk drive # OUTSEQ -1.0 9999.0 Output seq. no. Solution control adverbs: SOLINT Solution interval (min) APARM General parameters 1=min. no. antennas 2=min. SNR 3=min. ratio 4=max. ratio 5 >1=> avg. IF 6 >1=> divide only 7 >1=> tell solutions 8 >1=> input divided ANTWT Ant. weights (0=>1.0) UVRANGE Range of uv distance for full weight. 0 => all. WTUV Weight outside UVRANGE 0=0. BADDISK -1.0 1000.0 Disks to avoid for scratch. ---------------------------------------------------------------- FRCAL Type: Task Use: Determines ionospheric Faraday rotation corrections for a data set by comparison with a polarized model. Corrections are written into an SN table attached to the input uv data and a corrected uvdata set is written. NOTE: this task does NOT apply flagging or calibration tables to the input UV data. Run SPLIT first if that operation is desired. Adverbs: INNAME.....Input UV data file (name). Standard defaults. INCLASS....Input UV data file (class). Standard defaults. INSEQ......Input UV data file (seq. #). 0 => highest. INDISK.....Input UV data file disk drive #. 0 => any. IN2NAME....Cleaned map name (name). Standard defaults. IN2CLASS...Cleaned map name (class). Standard defaults. Class names are assumed to be of the form '?xxxxx' where ? is Q and U. IN2SEQ.....Cleaned map name (seq. #). 0 -> highest. IN2DISK....Disk drive # of cleaned map. 0 => any. INVERS.....CC file version #. 0=> highest numbered version NITER......# CLEAN comps. to use for model. 0 => all. PMODEL.....A single component model to be used instead of a CLEAN components model; if abs (PMODEL(2 or 3) > 0 then use of this model is requested. PMODEL(1) = I flux density (Jy) PMODEL(2) = Q flux density (Jy) PMODEL(3) = U flux density (Jy) PMODEL(4) = V flux density (Jy) PMODEL(5) = X offset in sky (arcsec) PMODEL(6) = Y offset in sky (arcsec) NOTE: PMODEL takes precedence over an IN2NAME model. 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 disk number with space The following control how the solutions are done, if you don't understand what a parameter means leave it 0 and you will probably get what you want. SOLINT.....The solution interval (min.) 0 = data integration. APARM......General control parameters. APARM(1)...Minimum number of antennas allowed for a solution. 0 => 4 APARM(2)...The minimum allowed signal-to-noise ratio. 0 => 5 APARM(3)...Minimum magnitude of the ration of the data to the model to be used. 0 = 0.75 APARM(4)...Maximum magnitude of the ration of the data to the model. 0 = 1.5 APARM(5)...If > 0 then average solution in IF. APARM(6)...If > 0 then return the data divided by the model. APARM(7)...If > 0 then tell about solutions. APARM(8)...If > 0 then the input data has already been divided by the model and only solutions will be determined. ANTWT......Antenna weights. These are additional weights to be applied to the data before doing the solutions, one per antenna. Use PRTAN to determine which antenna corresponds to each antenna number. 0 => 1.0 UVRANGE....The range of uv distance from the origin in kilowavelengths over which the data will have full weight; outside of this annulus in the uv plane the data will be down weighted by a factor of WTUV. 0 => all. WTUV.......The weighting factor for data outside of the uv range defined by UVRANGE. BADDISK...This array contains the numbers of disks on which it is desired that scratch files not be located. BADDISK has no effect on input and output data. ---------------------------------------------------------------- FRCAL: Faraday rotation self calibration task. Documentor: W. D. Cotton, NRAO Related Programs: PCAL, FARAD Linearly polarized signals undergo a rotation of the orientation of the electric vectors as they propagate through a magnetized plasma; an effect know as Faraday rotation. The ionosphere can introduce substantial, time and source variable Faraday rotation into the signals from radio sources. The magnitude of the effect is determined by the integral of the electron density times the component of the magnetic field along the line of sight. This effect can therefore vary strongly with observing geometry as well as electron density. Faraday rotation increases rapidly with decreasing frequency and increasing solar activity. Task FARAD attempts to make a correction for ionospheric Faraday rotation based on external models or measurments of the ionospheric electron density. However, when this effect is significant this correction may be inadequate. Task FRCAL uses a polarized model of the source along with the observations to estimate and remove the effects of ionospheric Faraday rotation. As the model of the polarized emission will be corrupted by the Faraday rotation multiple iterations of imaging - Faraday self calibration may be needed. In addition, the polarization calibration may be adversely affected by variable Faraday rotation and the instrumental polarization (task PCAL) may need to be included in the Faraday rotation self calibration loop for the polarization calibrator. The total intensity calibrations should have adjusted all the parallel hand phases to the same reference antenna so that the residual Faraday rotation effect is the right-left phase difference at the reference antenna. A single right-left phase difference is needed at any given time. The results of FRCAL are 1) an SN table attached to the input uvdata and 2) a corrected set of uv data. The corrections in the SN table can, in principle, be applied to another source although this will only work if the two sources involved are near by on the sky and can be assumed to have the same Faraday rotation properties. FRCAL first divides the observed RL and LR correlations by the Fourier transform of the polarized model to remove the effects of source structure. The result will be all (1,0) for a perfect model and no Faraday rotation. The phase of the divided RL and LR data will be rotated in opposite directions by Faraday rotation. In each solution interval the estimate of the Faraday rotation is determined from the weighted average of the phase of the divided RL data and the conjugate of the divided LR data. Half of this phase is added to the R gain phases in the SN table to all antennas in that solution interval and half is subtracted from the L gain phases. When applied to the visibility data this will have no effect of parallel hand (RR and LL) data but will remove the estimate of the Faraday rotation from the cross (RL, LR) hand data. Note: this procedure will only work if the parallel hand data is already fully calibrated and only the time variable right-left phase difference remains to be determined. Also any prior R-L offset calibration will be lost.