; LPCAL ;--------------------------------------------------------------- ;! Determines instrumental polarization for UV data ;# TASK UV CALIBRATION POLARIZATION ;--------------------------------------------------------------- ;; Copyright (C) 1995-2003, 2006-2008 ;; 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 ;----------------------------------------------------------------------- LPCAL LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC LPCAL Task to compute instrumental polarization corrections 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 # Data selection (multisource): CALSOUR Source to calibrate with TIMERANG Time range to use. SELBAND Bandwidth to select (kHz) SELFREQ Frequency to select (MHz) FREQID Freq. ID to select. BCHAN 0.0 2048.0 Start channel 0=>all ECHAN 0.0 2048.0 End channel 0=>all BIF 0.0 100.0 Lowest IF number 0=>all EIF 0.0 100.0 Highest IF number 0=>all ANTENNAS Antennas to solve for. UVRANGE 0.0 UV range in kilolamdba SUBARRAY 0.0 1000.0 Subarray, 0=>1 Cal. info for input: DOCALIB -1.0 101.0 > 0 calibrate data & weights > 99 do NOT calibrate weights GAINUSE CAL table to apply. CLEAN map. If not given, unpol. source is assumed. BLVER BL table to apply. FLAGVER Flag table version DOBAND -1.0 10.0 If >0 apply bandpass cal. Method used depends on value of DOBAND (see HELP file). BPVER Bandpass table version SMOOTH Smoothing function. See HELP SMOOTH for details. IN2NAME Cleaned map name (name) IN2CLASS Cleaned map name (class) IN2SEQ 0.0 9999.0 Cleaned map name (seq. #) IN2DISK 0.0 9.0 Cleaned map disk unit # IN2VERS -1.0 255.0 First CC file version #. NCOMP Number of sub-models, 1 value per field NMAPS 0.0 4096.0 No. Clean map files (fields) SOLINT Preavg. interval (min) 0 => no averaging. PRTLEV -1.0 10.0 Print statistics 0=>results 1=>results&residuals CPARM Task enrichment parameters (1) > 0 => average in IF and find common solution. (2) > 0 => compute model using the image BADDISK 0.0 1000.0 Disk no. not to use for scratch files. --------------------------------------------------------------- LPCAL Task: This task reads a data UV file and a total intensity source model and determines the effective feed parameters for each antenna and IF. These parameters are placed in the antenna (AN) table. Polarization corrections can then be applied by setting DOPOL=1 in LISTR, VBPLT, POSSM, or SPLIT. Model images made with both values of IMAGR's DO3DIMAG option are handled correctly. 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. The following are used for multisource data files only: CALSOUR....Source to use for feed calibration. Only one can and will be used. Must be set for multisource files. TIMERANG...Time range of the data to be used. In order: Start day, hour, min. sec, end day, hour, min. sec. Days relative to ref. date. SELBAND....Bandwidth of data to be selected. If more than one IF is present SELBAND is the width of the first IF required. Units = kHz. For data which contain multiple bandwidths/frequencies the task will insist that some form of selection be made by frequency or bandwidth. SELFREQ....Frequency of data to be selected. If more than one IF is present SELFREQ is the frequency of the first IF required. Units = MHz. FREQID.....Frequency identifier to select (you may determine which is applicable from the OPTYPE='SCAN' listing produced by LISTR). If either SELBAND or SELFREQ are set, their values override that of FREQID. However, setting SELBAND and SELFREQ may result in an ambiguity. In that case, the task will request that you use FREQID. BCHAN......First channel to use. 0=>1 ECHAN......Highest channel to use. 0=>all higher than BCHAN BIF........First IF to process. Old values for feed parameters and calibrator polarizations for unprocessed IFs are unchanged. 0=>all. EIF........Highest IF to process. 0=>all higher than BIF ANTENNAS...A list of the antennas to have solutions determined. If any number is negative then all antennas listed are NOT to be used to determine solutions and all others are. All 0 => use all. The following may be used for all data files (except as noted): UVRANGE....Range of projected spacings to be included in 1000's of wavelengths. 0 => 1, 1.E10 SUBARRAY...Subarray number to use. 0=>1. 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. BLVER......Version number of the baseline based calibration (BL) table to appply. <0 => apply no BL table, 0 => highest. FLAGVER....Specifies the version of the flagging table to be applied. 0 => highest numbered table. <0 => no flagging to be applied. DOBAND.....If true (>0) then correct the data for the shape of the antenna bandpasses using the BP table specified by BPVER. The correction has five modes: (a) if DOBAND=1 all entries for an antenna in the table are averaged together before correcting the data. (b) if DOBAND=2 the entry nearest in time (including solution weights) is used to correct the data. (c) if DOBAND=3 the table entries are interpolated in time (using solution weights) and the data are then corrected. (d) if DOBAND=4 the entry nearest in time (ignoring solution weights) is used to correct the data. (e) if DOBAND=5 the table entries are interpolated in time (ignoring solution weights) and the data are then corrected. BPVER......Specifies the version of the BP table to be applied. <0 => no bandpass correction done. SMOOTH.....Specifies the type of spectral smoothing to be applied to a uv database . The default is not to apply any smoothing. The elements of SMOOTH are as follows: SMOOTH(1) = type of smoothing to apply: 0 => no smoothing To smooth before applying bandpass calibration 1 => Hanning, 2 => Gaussian, 3 => Boxcar, 4 => Sinc To smooth after applying bandpass calibration 5 => Hanning, 6 => Gaussian, 7 => Boxcar, 8 => Sinc SMOOTH(2) = the "diameter" of the function, i.e. width between first nulls of Hanning triangle and sinc function, FWHM of Gaussian, width of Boxcar. Defaults (if < 0.1) are 4, 2, 2 and 3 channels for SMOOTH(1) = 1 - 4 and 5 - 8, resp. SMOOTH(3) = the diameter over which the convolving function has value - in channels. Defaults: 1,3,1,4 times SMOOTH(2) used when input SMOOTH(3) < net SMOOTH(2). IN2NAME....Cleaned map name (name). If not given, LPCAL assumes that the source is unpolarized. Note: a CLEAN image may be given only for one source. An Ipol, Lpol or Rpol image is expected. If the source table contains a flux, then that flux will be used to scale the components model to obtain the stated total flux. This is needed since initial Cleans may not obtain the full flux even though they represent all the essentials of the source structure. IN2CLASS...Cleaned map name (class). IN2SEQ.....Cleaned map name (seq. #). This must be the same for all fields. IN2DISK....Disk drive # of cleaned map. 0 => any. IN2VERS....CC file version # for the first sub-model. Applies to all fields. 0 => 1. NCOMP......THIS ADVERB IS USED IN A NON-STANDARD WAY IN LPCAL: Number of regions within the facet, each supported by a separate well-prepared Clean components file, to be used. The CC file version numbers to be used are IN2VERS through IN2VERS+NCOMP(i)-1 for sub-models 1 through NCOMP(i) in facet i. There is a limit of 10 sub-models over all facets included. The polarization of each sub-model may be different and will be determined by LPCAL along with the instrumental parameters. Each sub-model CC file should be prepared with CCEDT or TABED or in the imaging to include only the region over which the source polarization is thought to be roughly constant. NMAPS......Number of image files to use for model. If more than one file is to be used, the NAME, CLASS, DISK and SEQ of the subsequent image files will be the same as the first file except that the LAST 3 or 4 characters of the CLASS will be an increasing sequence above that in IN2CLASS. Thus, if INCLASS='ICL005', classes 'ICL005' through 'ICLnnn' or 'ICnnnn', where nnn = 5 + NMAPS - 1 will be used. Old names (in which the 4'th character is not a number) are also supported: the last two characters are '01' through 'E7' for fields 2 through 512. In old names, the highest field number allowed is 512; in new names it is 4096. SOLINT.....Time interval to average data before determining correction in min. 0 => no averaging. PRTLEV.....If larger than 0.0, show fit residuals. CPARM......Task enrichment parameters: (1) If CPARM(1) is greater than 0 then data in the selected IFs will be averaged before doing the solutions. NOTE: the IFs must be phase coherent for this to be useful. (2) If CPARM(2)>0 will compute models from images rather than CLEAN components. The VISDFT subroutine doing this doesn't seem to be working correctly -> don't use. BADDISK....Disk numbers on which scratch files are not to be placed. --------------------------------------------------------------- LPCAL: Task to determine effective feed polarization parameters for VLBI arrays. Documentor: K. J. Leppanen Related Programs: CALIB, LISTR, SPLIT, CLCOR, CCEDT,PCAL,SPCAL PCAL, SPCAL or LPCAL? --------------------- PCAL was designed for the VLA and works well for unresolved of slightly resolved calibrators with uniform polarization. LPCAL was designed for VLBI and allows the use of calibrators that have significant structure and differential polarization across the source. It will also handle unpolarized calibrators. SPCAL was designed for spectral line experiments and solves for frequency dependent D-terms. Like PCAL SPCAL demands an unresolved calibrator. LPCAL ===== Polarization calibration of synthesis array visibility data consists of two distinct parts: 1) the determination of the effective response of the feeds to the incident radiation and the correction of the observations to the values which would have been obtained with perfect feeds and 2) the determination and removal of systematic phase delay differences between the right and left hand polarization systems. LPCAL determines the effective response of the feeds and stores this information in the AN table. Routines which can apply calibration tables can then be instructed to apply the polarization corrections by setting the adverb DOPOL=TRUE. LPCAL can be used in two cases: 1) the polarization structure of the calibrator is not known, but a total intensity model for it is available and can be used to derive the polarization structure in the way explained below; 2) the calibrator is unpolarized. In the first case the parallactic angle range for the calibrator has to be sufficient to allow for the solution of source polarization. For a bright unpolarized calibrator even one scan should be sufficient and no total intensity model is required. LPCAL uses a linear approximation for the feeds, which becomes inaccurate if the polarization leakage in the feeds (D-terms) exceeds 5 percent. This should not be the case for new instruments like the VLBA. Note that the parallactic angles of the source must differ at different interferometer elements. This means that VLA data cannot be calibrated with LPCAL. A source with unknown polarization structure can be used as a calibrator if its polarization structure can be modeled as a weighted sum of a small number of sub-models, derived by simply splitting the total intensity (Stokes I) CLEAN model. The weights in the sum are initially unknown complex values that are solved for during the feed solution. Such a model is likely to be a good representation of the polarization structure of maser sources, which consist of distinct masing regions. It is often an adequate assumption for continuum sources that can be well modeled by a small number (say, less than five) of gaussians. STEPS BEFORE RUNNING LPCAL -------------------------- Before fringe fitting, apply the correction for parallactic angle using CLCOR, OPCODE='PANG'. Fringe fit the parallel hand visibilities using normal procedures for multiband data making sure that all solutions are referenced to the same antenna. Don't let FRING rereference the solutions if a change in the reference antenna cannot be avoided; i.e. set DPARM(7) = 1. Instead, use SNSMO to rereference the solutions. To find R-L phase and single-band delay offsets between the IFs, use BLAVG, FRING, and POLSN as described in the BLAVG help file. Self-calibrate the parallel hand data; this will also yield a total intensity source model for LPCAL. If the systematic phase delay difference between the right and left hand systems is time variable (e.g. variable ionospheric Faraday rotation at the reference station used in fringe fitting) then the phase drift needs to be removed before running LPCAL (see CLCOR). If the systematic phase offsets are essentially constant then they may be removed after running LPCAL. To do this, first use LISTR with STOKES='POLC'; DOPOL=1 and appropriate calibration on a source with known polarization angle to determine the phase offset and then apply a correction using CLCOR and OPCODE='POLR'. Note: this will modify the AN table (rotate the D-term phases) as well as the CL table. TOTAL INTENSITY MODEL --------------------- For a calibrator with unknown polarization, LPCAL needs a total intensity source model, which is usually given as a set of sub-models (CC tables). The division into sub-models can be made with the task CCEDT automatically or manually. The division should be made in such a way that the polarized structure of the resulting sub-models is well approximated by their (scaled) total intensity structure with constant polarization angle (similarity assumption). Usually one would use a Stokes I model, but Stokes L or R models are also accepted. It is crucial that the sum of the total intensity sub-models gives a good fit to the parallel-hand visibilities. Otherwise there is no hope that it can be the basis for a successful model of the polarization structure. Example 1: In case of H2O or SiO masers, each significant maser spot should constitute an independent sub-model, since each can have a different degree and angle of linear polarization. These models can be given as separate CC tables, obtained by splitting the original CC table with CCEDT. If the source has very extended structure it may be necessary to map it using multiple fields in MX. LPCAL can read CLEAN models from multiple fields, each having one or more CC tables (again, one per sub-model). Give the number of fields in NMAPS, the CC version number of the first model in IN2VERS (applies to all fields), and the numbers of consecutive CC models to use in NCOMP (one value per field). Example 2: If using a continuum calibrator, the preferable way to produce the sub-models may be the automatic splitting feature of CCEDT. Try running CCEDT with CPARM=10,0 on the CC table produced by IMAGR or MX. For manual splitting with CCEDT (and TVBOX, for example), it may be useful to make a heavily super-resolved image to see how the CLEAN components tend to group. RUNNING LPCAL ------------- IN2NAME, IN2CLASS, IN2SEQ, IN2DISK: These specify the total intensity model to use. A total intensity model is irrelevant if emission from all parts of the source is unpolarized, in which case these should be left empty. Note, however, that even then the input uv-data must have been self-calibrated, i.e., the complex gains must be correct. IN2VERS, NCOMP, NMAPS: See example 1 above. SOLINT: The data will be preaveraged to SOLINT minutes before the solution. This should be set sufficiently short to avoid smearing of source structure. For extended maser sources 0.5 minutes can be too much! Regardless of SOLINT, only one solution per station and possibly IF is made by LPCAL. PRTLEV: LPCAL will solve for the fractional linear polarization of each sub-model and print out the solutions. PRTLEV<0 suppresses this printing. Note that the source solutions are not saved anywhere. PRTLEV=1 (recommended) produces useful information about the residuals. SINGLE SOURCE FILES ------------------- In principle, LPCAL can work on a single source file followed by SPLIT to apply the correction. However, in practice, it is preferable to convert the input file to multisource file using task MULTI followed by INDXR to create an index table.