; IMSCAL ;--------------------------------------------------------------- ;! large image self-cal with IM2CC and OOCAL ;# PROCEDURE CALIBRATION AP UV OOP ;----------------------------------------------------------------------- ;; Copyright (C) 2016-2017 ;; 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 ;----------------------------------------------------------------------- IMSCAL LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC IMSCAL: Determines antenna complex gain from large image Input uv data. INNAME UV file name (name) INCLASS UV file name (class) INSEQ 0.0 9999.0 UV file name (seq. #) INDISK 0.0 9.0 UV file disk drive # input image IN2NAME Large image name (name) IN2CLASS Large image name (class) IN2SEQ Large image name (seq #) IN2DISK Large image name (disk) BLC 0.0 4096.0 Bottom left corner of image 0=>1 TRC 0.0 4096.0 Top right corner of image 0=>max allowed ICUT 0.0 Include all points > ICUT in absolute value only FLUX Discard all points < FLUX NX 1.0 Number panels in X NY 1.0 Number panels in Y OUTNAME name to use for temporary files (CC, OOSUB output) OUTDISK Disk to put subimages/CCs Solution control adverbs: DOKEEP -1.0 1.0 > 0 -> keep divided us data Data selection (multisource): ICHANSEL Array of start and stop chn numbers, plus a channel increment and IF to be used for channel selection in the averaging. See HELP ICHANSEL. Default = center 75% of band. ANTENNAS Antennas to select. 0=all DOFIT Subset of ANTENNAS list for which solns are desired. ANTUSE Mean gain is calculated (CPARM(2)>0) using only the listed antennas. See explain. UVRANGE Range of uv distance for full weight WTUV Weight outside UVRANGE 0=0. WEIGHTIT 0.0 3.0 Modify data weights function Cal. info for input: CMETHOD Modeling method: 'DFT','GRID',' ' REFANT Reference antenna SOLINT Solution interval (min) SOLSUB Solution subinterval SOLMIN Min solution interval APARM General parameters 1=min. no. antennas 3 > 0 => avg. RR,LL 5 > 0 => avg. IFs. 6=print level, 1=good, 2 closure, 3 SNR 7=SNR cutoff (0=>5) 8=max. ant. # (no AN) 9 > 0 => pass failed soln 10 < 99 cal output weights Phase-amplitude Parameters: DOFLAG Flag on closure error? SOLTYPE Soln type,' ','L1','GCON', 'R', 'L1R', 'GCOR' SOLMODE Soln. mode: 'A&P','P','P!A', 'GCON', SOLCON Gain constraint factor. MINAMPER 0.0 Amplitude closure error regarded as excessive in % MINPHSER 0.0 Phase closure error regarded as excessive in degrees NORMALIZ -1.0 4.0 >0 => normalize gain: 1 globally, 2 by subarray, 3 by subarray,IF 4 by subarray,IF,pol CPARM Phase-amp. parameters 1 = Min el for gain normalization (deg) 2 > 0 normalize w median else use mean 3 avg. amp. closure err 4 avg. ph. closure err 5 = 1 vector average channels, scalar avg between times >= 2 scalar average 6 limit clipping in robust 7 limit display of closure errors ANTWT Ant. weights (0=>1.0) GAINERR Std. Dev. of antenna gains. BPARM Task enrichment parameters (1) Antenna diameter (m) 0 -> no correction (2) Omit CC options (3) spectral index radius 0 -> no correction FQTOL Frequency tolerance in kHz (primary beam & spec index) IN3NAME Spectral index image name IN3CLASS Spectral index image class IN3SEQ Spectral index image sequence number IN3DISK Spectral index image disk IN4NAME Spectral curvature name IN4CLASS Spectral curvature class IN4SEQ Spectral curvature sequence number IN4DISK Spectral curvature disk BADDISK Disk no. not to use for scratch files. ---------------------------------------------------------------- IMSCAL Task: IMSCAL is a procedure that combines IM2CC, OOSUB, CALIB, and finally TACOP. IM2CC breaks a large (usually CASA) image into pieces, making both an image and a Clean components table for each piece. This is needed to allow AIPS to use its geometries while CASA has used the W-projection geometry. OOSUB divides the data for a calibration source by the model from these pieces plus options to permit frequency-dependent primary beam and spectral index corrections. CALIB then determines an IF-dependent, spectral-channel independent antenna gain and phase, using the divided data. A solution (SN) table is written and copied to the input uv file by TACOP. Finally the temporary, divided data set is deleted (optionally) and the image pieces are deleted.. This procedure does not apply data selection and calibration adverbs to the input data set. You must apply these with SPLIT or SPLAT (or other tasks) to make a data set consisting solely of the edited/calibrated data that you wish to self-cal. IMSCAL makes a number of temporary files all of which are assigned specific names including sequence number 1. It will check for the presence of any of these on your disks before doing work. If any are present, IMSCAL will list them and quit, allowing you to rename them or delete them. This procedure is obtained by entering RUN OOCAL. 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. **** Model image in Jy/pixel (not convolved with beam) **** IN2NAME....Input image file name (name). Standard defaults. IN2CLASS...Input image file name (class). Standard defaults. IN2SEQ.....Input image file name (seq. #). 0 -> highest. IN2DISK....Disk drive # of input image. 0 -> any. BLC........The bottom left-hand pixel of the input image which becomes the bottom left corner of the subimage from which the NX x NY panels are taken. 0 -> 1. TRC........The top right-hand pixel of the input image which becomes the top right corner of the subimage from which the panels are taken. 0 -> max allowed value. ICUT.......CC components are made only from pixel values greater in absolute value than ICUT and FLUX.......CC components are made only from pixel values greater than FLUX (in actual value). Thus FLUX=0 cuts off all negatives. NX.........The X axis is divided into NX nearly equal panels. NY.........The Y axis is divided in NY nearly equal panels. Be sure to make enough to account for any W term issues. OUTNAME....The output subimages are stored on OUTDISK with this name parameter. The OUTSEQ=1 and OUTCLASS=IMCnnn. The output from the OOSUB step in OOCAL also uses this name with OUTCLASS 'OOCAL1'. OUTDISK....The output subimages and CC files are put on OUTDISK, the OOSUB output file is put on INDISK. It is better to avoid Lustre disks for the CC files. DOKEEP.....> 0 => keep the file produced by OOSUB containing the input data divided by the model and the SN table produced by CALIB <=0 => delete this temporary file after TACOP. The following may be used for all data files (except as noted): ICHANSEL...Array of start and stop channels plus a channel increment and IF, used to select the channels to be averaged. For instance, if you wished to exclude channels 1 - 10 and 121 - 128 because of bandpass effects, and channels 56 - 80 of IF 1 but not IF 2 because of interference, then you would set ICHANSEL = 11,55,1,1, 81,121,1,1, 11,121,1,2. If you only wished to use every other channel from the second IF then you would set ICHANSEL = 11,55,1,1, 81,121,1,1, 11,121,2,2. Up to 20 groups of start, stop and increment channel numbers plus IF numbers can be specified. The default (ICHANSEL = 0) is to average the center 75% of the band, i.e. ICHANSEL(1,1) = (# channels)/8 + 1 For example: # channels=16 => ICHANSEL(1,1)=3 ICHANSEL(2,1) = (# channels + 1)*7/8 For example: # channels=16 => ICHANSEL(2,1)=14 ICHANSEL(3,1) = 1 ICHANSEL(4,1) = 0 (meaning all IFs). If ICHANSEL describes averaging explicitly for some IFs, but skips other IFs, then the center 75% of the band is averaged for the skipped IFs. For example: ICHANSEL=2,6,1,2 => The channels 2-6 will be averaged for IF=2 and the center 75% of the band will be averaged for the rest of the IFs. 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. DOFIT......A list of the antennas for which solutions should be determined. Only those antennas listed in DOFIT will be solved for; all data selected via ANTENNAS will be used to form the solutions. If any antenna number in DOFIT is <= -1, then DOFIT is taken as the list of antennas for which no solution is desired; a solution is found for all antennas not in DOFIT. Note that the REFANT, if specified, will _not_ be solved for even if it appears in the DOFIT list. Selection via DOFIT can be disabled by setting DOFIT = 0 which defaults to solving for all antennas. NOTE: THIS OPTION MUST NOT BE USED UNLESS YOU UNDERSTAND IT FULLY. Basically, it should be used to solve for the gains of "poor" antennas after the "good" antennas have been fully calibrated. Antennas included in ANTENNAS but not in DOFIT are assumed to have a complex gain of (1,0) and the gains produced will be very wrong if this is not the case. See HELP DOFIT. ANTUSE.....A list of the antennas to be used in the calculation of the mean gain modulus (NORMALIZ>0). If any number is negative then all antennas listed are NOT to be used to determine the gain normalization and all others are. All 0 => use all. It can be useful to limit the antennas used for the gain normalization to those with good a priori calibration, especially when using VLBI-style calibration based on system temperatures and gains. This prevents the flux scale from being dragged up or down by poorly calibrated antennas including antennas subjected to bad weather. The normalization factor determined using the ANTUSE antennas is applied to all antennas. SUBARRAY...Subarray number to use. 0=>all. 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. WTUV.......The weighting factor for data outside of the uv range defined by UVRANGE. WEIGHTIT...If > 0, change the data weights by a function of the weights just before doing the solution. Choices are: 0 - no change weighting by 1/sigma**2 1 - sqrt (wt) weighting by 1/sigma may be more stable 2 - (wt)**0.25 3 - change all weights to 1.0 CMETHOD....This determines the method used to compute the model visibility values. 'DFT' uses the direct Fourier transform, this method is the most accurate. 'GRID' does a gridded-FFT interpolation model computation. ' ' allows the program to use the fastest method. NOTE: when using a model derived from data with difference uv sampling it is best to use 'DFT' 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. REFANT.....The desired reference antenna for phases. SOLINT.....The solution interval (min.) 0 => scan average for multi-source, 0 => 10 s for single source amp-phase solns. (VLA) 0 => 10 min for delay-rate solutions (VLBA). OOCAL tries hard to make equal integrations within each scan but that is a problem that lacks a general solution. You can help by careful choice of SOLINT: assume you have data every 10 seconds. Then, to get 1 sample per solution, set SOLINT=9/60. To get 2 per solution, set SOLINT=19/60, 3 per solution SOLINT=29/60. Each averaged interval will start with an actual data sample and will end just before the first sample at a time greater than the start + SOLINT + 0.1s. At the end of the scan, the end time can be increased by up to 0.6 * SOLINT to prevent short final integrations. For calibration that is not self-calibration, note that the 2-point interpolation will use ONLY the last integration of a calibrator scan with the first integration of the next calibrator sacn. That is why the initial calibration normally uses scan averages for the calibrator sources. ------------------------- If the times in your data set are not at regular intervals due to flagging and averaging, you must be careful with SOLINT. To get all data in 10 seconds (from 0 through 9.999) set SOLINT to 9.999/60. Use of 1 sec will do odd things with the records at odd times. ------------------------- SOLSUB.....The begin time for the next interval in advanced from the current one by SOLINT / SOLSUB where 1 <= SOLSUB <= 10. 0 -> 1. This is to produce solutions at sub-intervals of SOLINT based on SOLINT length of averaging. SOLMIN.....Minimum number of subintervals to be used in a solution. 0 -> SOLSUB. APARM......General control parameters. APARM(1)...Minimum number of antennas allowed for a solution. 0 => max (3, min (6, Numant/2)) APARM(2)...If > 0 then the input data has already been divided by a model; only solutions will be determined. APARM(3)...If > 0 then average RR, LL APARM(5)...If > 0 then make a combined solution for the IFs; if <= 0 then make separate solutions. APARM(6)...Print flag, 0=none, 1=time, closure error statistics, 2=individual closure failures (exceeding both CPARM(7) sigma and either MINAMPER or MINPHSER), 3=some additional info including the antenna signal to noise ratio, 4=solutions, 5=data too. APARM(7)...The minimum allowed signal-to-noise ratio. 0 => 5 APARM(8)...If there is no antenna (AN) table with the input file then the maximum antenna number in the file should be entered in APARM(8). APARM(9)...When solutions fail or there is insufficient data and APARM(9) > 0 then (1,0) is written to the SN table. This will preserve the previous calibration but this option should be used with extreme care. APARM(10)..When writing a single-source output file, calibrate the output weights except when either DOCAL > 99 or APARM(10) > 99 or both. Phase-amplitude parameters: DOFLAG.....If DOFLAG > 0, those baselines with excessive closure error will be added to a new flag table along with all flags previously in FLAGVER. If DOFLAG <= 0, no new data flags are generated and no flag table is written. In either case, all SOLTYPEs below check the data for closure error and will report the fractions for which the closure error exceeds abs(DOFLAG) times the rms closure error. Defaults for the reporting level are: 0.0 < DOFLAG < 2.0 -> 2.5 -1.0 < DOFLAG <= 0.0 -> -2.5 if APARM(6) = 0 -99.0 if APARM(6) >= 1 Note that this checking of closure error is in addition to the checking done under control of MINAMPER, MINPHSER, and APARM(6) which do not flag the data. Note that data marked as "bad" by the DOFLAG test will not be checked with the APARM(6) controlled tests. The default on DOFLAG=0 is then meant to enable better printing of closure errors when not doing any actual flagging. SOLTYPE....Solution type: ' ' => normal least squares, 'R ' => as ' ' with robust iteration 'L1 ' => L1 solution; a weighted sum of the moduli of the residuals is minimized. The computed gain solutions are less influenced by wild data points, but there is some loss of statistical efficiency. See [F.R. Schwab, VLA scientific Memo #136] for further details. 'L1R ' => as 'L1' with robust iteration 'GCON' => least squares which may include gain constraint. 'GCOR' => as 'GCON' with robust iteration SOLTYPE (other than the R) is ignored when the DOFIT option is used. The robust versions iterate the solution, discarding data that does not fit the current solution well enough. They should be less disturbed by bad data, but will be slower. SOLMODE....Solution mode: 'A&P ' => amplitude and phase. 'P ' => phase only 'P!A ' => phase only (no amplitude information) 'GCON' => amplitude and phase with constraints on amplitude. This mode requires setting SOLTYPE='GCON', uses GAINERR and SOLCON may be used. ' ' => 'A&P ' for multisource (raw) data, => 'P ' for single source data. SOLCON.....Gain constraint factor; a value larger than 0 will increase the strength of the amplitude constraint in gain constrained solution with SOLMODE='GCON' MINAMPER...Amplitude closure error regarded as excessive in per cent. If APARM(6) > 0, summaries of the number of excessive errors by antenna are printed and, if APARM(6) > 1, up to 1000 of the individual failures are printed. 0 => do not check or report "excessive" closure errors of any sort. Note that amplitude closure errors are accumulated using logarithms so that gains of 0.5 and 2.0 are both errors of 100%. MINPHSER...Phase closure error regarded as excessive in degrees. If APARM(6) > 0, summaries of the number of excessive errors by antenna are printed and, if APARM(6) > 1, up to 1000 of the individual failures are printed. 0 => do not check or report "excessive" closure errors of any sort. NORMALIZ...If > 0, constrain the mean gain modulus of the calibration applied to be unity. This is mostly used in self calibration. If NORMALIZ = 1, the mean is over all IFs, antennas, polarizations and subarrays. If NORMALIZ =2, it is the same except separated by subarray. If NORMALIZ = 3, it is averaged over all antennas and polarizations but separated by IF and subarray. NORMALIZ=4 also separates by polarization. If you select NORMALIZ= 1, the global scaling factor is written to the header of the SN table. For options 2 - 4, the SN table is re-written with the final scaling applied. If = 0, substitute CPARM(2) which was previously used for this information. CPARM......Phase-amplitude parameters. CPARM(1)...Minimum elevation in degrees for the solutions used to constrain the mean gain modulus. 0 or >80 => no constraint (actually -100 is used). CPARM(2)...If NORMALIZ > 0, CPARM(2) controls whether a mean value (CPARM(2) <= 0) or a median value (CPARM(2) > 0) is used. CPARM(3)...If > 0, the values of any amplitude closure errors whose average absolute percentage value exceeds CPARM(3) will be printed if APARM(6) > 0. CPARM(4)...If > 0, the values of any phase closure errors whose average absolute value exceeds CPARM(4) degrees will be printed if APARM(6) > 0.. CPARM(5)...If > 0 then the amplitudes will be scalar averaged when averaging across times before determining the solutions. The averaging of spectral channels will be a vector average unless CPARM(5) > 1.5. Vector averaging is preferred to avoid the Ricean bias in amplitudes, but not when phase instability will make the signal incoherent.If the atmospheric phase is very unstable, then it should be fine at any one time but may require the scalar averaging between times. CPARM(6)...The robust solution method discards data more than f(iter) * rms(iter) from the current solution to find the iter+1 solution f(iter) = max (g(iter), CPARM(6)) and g = 7.0, 5.0, 4.0, 3.5, 3.0, 2.8, 2.6, 2.4, 2.2, 2.5. Thus CPARM(6) can be used to limit the discarding to less restrictive values. CPARM(7)...The printing of individual closure errors occurs only if APARM(6) >= 2 and the errors exceed MINAMPER percent in amplitude and/or MINPHSER degrees in phase. That printing is also limited to those errors that are more than CPARM(7) times the 1 sigma expected error (based on the data weights). 0 -> 2.5. If you want no limit, set CPARM(7) to something like 0.001. 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 GAINERR....Estimates of the standard deviation of the modulus of the gains for each antenna. These are used ONLY if SOLMODE and SOLTYPE='GCON'. The solution will attempt to make the standard deviation of the modulus of the antenna gains match these values so accurate values are essential. BPARM......Correction control parameters (SEE EXPLAIN IMAGR): (1) If > 0 then make frequency dependent primary beam corrections assuming an antenna diameter of IMAGRPRM(1) meters. Note that VLA and ATCA arrays (TELESCOPE header parameter) use the default primary beam parameters defined elsewhere in AIPS, while other antennas actually use IMAGRPRM(1) as the diameter of a "standard" telescope. See FQTOL below also. (2) If BPARM(1) > 0, you may omit selected CCs from the operation based on position: BPARM(2) <= 0 : Include all CCs = 1 : Omit CCs within the main beam at all frequencies = 2 : Omit CCs within the main beam at some frequncies = 3 : Omit Ccs outside the main beam at some frequencies = 4 : Omit CCs outside the main beam at all frequencies (3) 1 => use a spectral-index image represented in IN3NAME, IN3CLASS, IN3SEQ, IN3DISK below to correct the Clean component model for each channel. IN4NAME et al will also be used as a curvature image iff IN3NAME are specified. BPARM(3)-0.5 is used as a radius in pixels over which the spectral index image is averaged. When it is small (0 < BPARM(3) <~ 1), the spectral index is interpolated rather than averaged. See FQTOL below as well. When doing spectral index, the primary beam correction (BPARM(1)) costs very little extra. This parameter is IMAGRPRM(17) in IMAGR. FQTOL......Frequency tolerance in kHz. Spectral channels with FQTOL are handled together (use the same average CC model) when applying the primary beam and spectral index corrections. Default is to do each channel separately which can take a long time. IN3NAME....Image name of spectral index image; no default. IN3CLASS...Image class of spectral index image; no default. IN3SEQ.....Image sequence of spectral index image; 0 -> highest. IN3DISK....Disk of spectral image image; 0 -> any. IN4NAME....Image name of spectral index curvature image; no default. Curvature images should be base 10 rather than base e - they differ by a factor of 2.3. Also the reference frequency is 1.0 GHz. These are changes done 2010-07-13. IN4CLASS...Image class of spectral index curvature image; no default. IN4SEQ.....Image sequence of spectral index curvature image; 0 -> highest. IN4DISK....Disk of spectral curvature image image; 0 -> any. BADDISK....Disk numbers on which scratch files are not to be placed. ---------------------------------------------------------------- OOCAL: Task to determine antenna gains from calibrator data Documentor: A.H.Bridle Related Programs: CLCAL, LISTR, SPLIT, UVFLG This task is the central AIPS routine for calibrating multi- source uv data sets using observations of calibration sources that can either be assumed to be point sources or have well determined structures. OOCAL determines antenna voltage gain solutions (amplitude and/or phase) from data for calibrator sources with well known flux densities, positions and structures. It is the equivalent of the VLA Dec-10 ANTSOL (with additional options) and of a combination of the AIPS routines ASCAL and VSCAL. Solutions determined by OOCAL under control of the APARM, CPARM and DPARM parameters are written to the solution (SN) extension table of the input uv data set. Solution tables may be merged, smoothed and interpolated into calibration (CL) tables for multi-source uv data files using CLCAL. OOCAL may also be run on files containing data for only one source, for self calibration. To run OOCAL, you should specify at least: The input uv data file (INNAME, INCLASS, INSEQ, INDISK). The CALSOURces to be used for determining antenna gains, or leave CALSOUR blank and specify a CALCODE and/or qualifier. REFANT, the reference antenna for the solution (choose an antenna with good signal to noise that was present through as much of the observing as possible). The defaults are set so that running OOCAL on a multi- source uv data file setting only these inputs will make a solution file for all IFs in the data over the entire time range using the highest-numbered flag file. All antennas will be calibrated for amplitude and phase, using data from the entire uv range. All antennas will be equally weighted. Point source models will be assumed for the calibration sources. The solution will be written to an SN table. Other useful options ==================== Use APARM(6)=3 to list the signal to noise ratio at each antenna solution. Solutions with signal to noise below 5:1 are probably not meaningful and will be discarded by the default setting of APARM(7). You may wish to apply more stringent criteria with APARM(7). Use SOLTYPE and SOLMODE='' to solve for both amplitude and phase solutions simultaneously with no constraints on amplitude, SOLTYPE and SOLMODE='GCON' for amplitude and phase with constraints set by GAINERR and SOLCON. Amplitude solutions for point source models in multisource files will be based on the flux densities entered for the sources in the source (SU) table extension of the data set using task SETJY. Use UVRANGE and WTUV to weight different uv ranges differently (or to restrict the solution to some uv range -- WTUV = 0 is read as zero weight). "CLEAN" COMPONENT MODELS ======================== OOCAL does not restrict you to the use of point source models for your calibrators. Use IN2DISK, IN2NAME, IN2SEQ, INVERS, NCOMP and NMAPS to specify a CLEAN component model for the field or fields around a calibrator and specify that calibrator in SOURCE. OOCAL will create an SN table for that calibrator alone. This SN table may then be merged with SN tables for other calibrators produced by other runs of OOCAL, when the SN tables are smoothed and interpolated into a CL file by CLCAL. FLUX CALIBRATOR MODELS ====================== This is actually a subsection of CLEAN COMPONENT MODELS above. You are strongly encouraged to use the flux calibrator models available for all the primary flux calibrators (3C138, 3C147, 3C286 and 3C48). Using calibrator models removes the need set UVRANGE and ANTENNAS. To see what calibrator models are available in AIPS type CALDIR, to read them in use the task CALRD. CALRD loads in the selected model as an image file. Then specify this image in IN2DISK, IN2NAME, IN2CLASS and IN2SEQ. OOCAL will recognize these images as standard calibrator models and scale the clean components in the central part of the field with the flux in the SU table. RESETTING YOUR CALIBRATION ========================== The first version of the CL table attached to your uv data set is protected from modification in CLCAL, so that you can easily "undo" all calibration steps that have taken place within AIPS. To reset your calibration, delete all CL tables with version numbers >1, and delete all SN tables. POLARIZATION considerations (thanks to Robert Braun) ==================================================== The VLA measures approximately (neglecting the leakage terms): RR=I+V, LL=I-V, RL=Q+iU, LR=Q-iU The WSRT uses linear polarization but the equatorial mounts mean that the feed orientation remains constant relative to the sky (no parallactic angle change at all). Thus, the WSRT measures approximately: XX=I-Q, YY=I+Q, XY=-U+iV and YX=-U-iV If you compare these two sets of equations, you see that they have a lot in common. If you simply pretend that you have measured (RR,LL,RL,LR) by changing the Stokes value from -5 to -1 in the header with PUTHD, you're almost in business, except you have -Q in the place of V, -U in the place of Q, and V in the place of U. This is fine for most things, since you just have to request a slightly different parameter from the one you really mean. The biggest hassle comes from amplitude calibration of linearly polarized sources, like: 3C286 (near 1.4 GHz) which has (I,Q,U,V)=(14.65,0.56,1.26,0.00) Jy 3C138 (near 1.4 GHz) which has (I,Q,U,V)=(8.30,0.63,-0.17,0.00) Jy Since Q is non-zero, it means that the XX and YY correlations do not correspond to the same real flux density (ie. XX=I-Q and YY=I+Q). Now, since RR=I+V and LL=I-V, the idea is to have OOCAL use the Stoke's V from the SU table. The, for WSRT data, one can fudge the right behavior, by putting the source's actual value of -Q in place of V in the SU table. This shouldn't harm any VLA users, since Stoke's V is near enough zero for most sources anyway. And if it were non-negligible it should be taken along in any case, since it really does affect the RR and LL correlations, and therefore the derived gain of the R and the L IFs.