; CLIP ;--------------------------------------------------------------- ;! deletes UV data with amplitudes outside specified range ;# Task UV EDITING ;----------------------------------------------------------------------- ;; Copyright (C) 1995-1996, 1999 ;; 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 ;----------------------------------------------------------------------- CLIP LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC CLIP Task to flag data whose amplitude is out of range. 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 # 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 #. BCHAN 0.0 4096.0 Begin channel number. ECHAN 0.0 4096.0 End channel number. BIF First IF to consider EIF Highest IF to consider UVRANGE 0.0 UV range in which to apply clip (kilo wavelengths) ANTENNAS Antennas to consider 0=>all BASELINE Baselines with ANTENNAS APARM Flux range by polarization: (1) = max. allowed parallel (2) = max. allowed cross-pol. (3) = min. allowed parallel (4) = min. allowed cross-pol. (5) > 0 => convert to Stokes before testing amplitudes (6) > 0 => do not flag cross because parallel flagged (7) > 0 => keep all data (8) > 0 => flag all channels if >= APARM(8) are clipped PRTLEV ABS(PRTLEV) clipped UV points printed ---------------------------------------------------------------- CLIP Task: This task will flag all visibilities whose amplitudes are outside some range. If a parallel polarization is flagged, all corresponding cross polarized visibilities will also be flagged unless APARM(6) > 0. The number of visibilities flagged is reported by the task. CLIP normally works on the individual correlator values, but the test can be performed on the Stokes values if APARM(5) > 0. (NOTE: UVPLT is normally used to plot Stokes I making the combination a bit confusing.) 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. 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 with space. BCHAN......Begin (line) channel number. 0 => 1. ECHAN......End (line) channel number. 0 => highest. BIF........Lowest numbered IF to consider. 0 => 1. EIF........Highest numbered IF to consider. 0 => highest. UVRANGE....Range of projected spacings in kilo wavelengths over which the task looks for excess amplitudes. ANTENNAS...A list of the antennas to be copied. All 0 => all. If any number is negative then all antennas listed are NOT to be copied and all others are. BASELINE...If any elements of BASELINE are non zero then all baselines between antennas named in ANTENNAS and those named in BASELINE are specified. If ANTENNAS contains a negative value then all baselines NOT specified are selected. APARM......Sets an allowable range in amplitudes (Jy) APARM(1) = max. parallel-hand polarization. (IPOL, RR, or LL) 0 => 1.e6 APARM(2) = max. cross-polarization. 0 -> 1e6. (QPOL, UPOL, VPOL, RL, or LR) APARM(3) = min. parallel-hand polarization. APARM(4) = min. cross-polarization. Note that sign is significant for autocorrelation data such as single-dish data. APARM(5) > 0 => convert to Stokes before testing the amplitudes. APARM(6) > 0 => do not flag cross-polarization correlators when the corresponding parallel-hand ones are flagged. APARM(7) > 0 => keep fully flagged data records (else they are omitted from output) APARM(8) > 0 => Flagg all spectral channels of an IF/Stokes if >= APARM(8) of them are clipped. PRTLEV..... ABS(PRTLEV) clipped UV-data points will be printed. If PRTLEV is < 0, then Real, Imaginary and Weight of UV data are also printed. ---------------------------------------------------------------- CLIP: Task to flag data whose amplitude is out of range. Related Programs: PRTUV, UVFLG, UVSUB, UVPLT PURPOSE CLIP is a task which will flag all visibilities whose amplitudes are larger than a specified limit or are smaller than a specified limit. Some interaction between the flagging of correlators is possible. The task is the most convenient method for flagging large, spurious visibility values. Before serious imaging it is worthwhile to run UVPLT in order to display the visibility amplitude as a function of UV projected spacing. The plot should show a relatively smooth decline of amplitude versus spacing; the details of which depend of the angular extent of the source and the signal to noise. Large amplitude spikes are generally very obvious and these visibilities can be flagged by using CLIP over all or some the visibility spacing range. NOTE: UVPLT is normally run to display Stokes I - the vector average of RR and LL. This vector average can appear ok with bad, high amplitude RR and LL of different phase or can appear low (and bad) with ok amplitude RR and LL of different phase. CLIP will flag the former, but not the latter in its normal mode. However, an option allows the conversion to Stokes in CLIP before testing the amplitudes. For normal VLA data, if IPOL or VPOL is bad, RR and LL will be flagged (and RL and LR depending on APARM(6)) and, if QPOL or UPOL is bad, RL and LR will be flagged. For a bright radio source where the correlated flux density in the source obscures possible amplitude spikes, the following scheme is advised. After getting a reasonably CLEAN image, subtract the clean components from the visibility data using UVSUB. The data will now be more dominated by noise since the majority of flux density from the source has been removed, and the subsequent use of UVPLT and CLIP can flag bad data. Do not forget to add back in the clean components using UVSUB in the opposite sign in order to get back to the original visibility data. After execution CLIP will report on the number of visibilities which have been flagged. The correlator order (1,2,3,4) corresponds to (RR,LL,RL,LR). This report is placed into the History file. COMMENTS OUTNAME, OUTCLASS, OUTSEQ, OUTDISK If the name describing the output visibility file is identical to the name of the input visibility file, the clipped data set will overwrite the input data set. This is somewhat dangerous unless the input data set is backed up, but does save a lot os disk space if necessary. UVRANGE Set the uv-spacing range (in klambda) appropriately for sources which are dominated by large and small-scale structure. APARM (1) Clip any parallel hand vis. amplitude greater than this value. 0 => no upper limit. (10**6) (2) Clip any crossed-hand vis. amplitude greater than this value. 0 => no upper limit. (10**6) (3) Clip any parallel hand vis. amplitude less than this value. 0=>no lower limit. (-10**6) (4) Clip any crossed-hand vis. amplitude less than this value. 0=>no lower limit. (-10**6) Sign is significant - autocorrelation data may have either sign while cross-correlation data always has positive amplitude. (5) <= 0 => Apply clip limits to the individual correlators found in the input data base. > 0 => Convert to Stokes visibilities, apply clip limits and flag the corresponding correlators. (6) <= 0 => Flag crossed-hand correlators if parallel hand correlator(s) are flagged. > 0 => Do not flag crossed-hand correlators if parallel hand correlator(s) are flagged. (7) <= 0 => Do not pass full flagged records to the output file. > 0 => Pass all records to the output, even if fully flagged. (8) > 0 => Flag all spectral channels of a given IF and Stokes if APARM(8) or more of them exceed the clip limit. <= 0 => Flag spectral channels individually. PRTLEV Gives the number of UV data points clipped to be printed. If pRTLEV is positive only the Visibility number, U, V, two baseline antennas and the time (in days) are printed; i.e., if PRTLEV = 1 then the following is printed CLIP VIS NUM U(LAMBDA) V(LAMBDA) A1 A2 TIME(DAYS) CLIP 12 1.18E+05 2.84E+05 6 10 3.14931E-01 If PRTLEV =-1 then the real, imaginary and weights are printed for all STOKEs and IFs, but only for the first spectral CHANNEL (BCHAN). Below is a 4 Stokes, 2 IF output: CLIP VIS NUM U(LAMBDA) V(LAMBDA) A1 A2 TIME(DAYS) CLIP 12 1.18E+05 2.84E+05 6 10 3.14931E-01 CLIP IF STOKE REAL IMAGINARY WEIGHT CLIP 1 1 9.44E-01 -4.53E-02 -1.0E+00 CLIP 1 2 9.28E-01 -8.85E-03 -1.0E+00 CLIP 1 3 3.64E-03 6.85E-02 -1.0E+00 CLIP 1 4 1.65E-02 -5.34E-02 -1.0E+00 CLIP 2 1 0.00E+00 0.00E+00 0.0E+00 CLIP 2 2 9.93E-01 -2.59E-02 1.0E+00 CLIP 2 3 0.00E+00 0.00E+00 0.0E+00 CLIP 2 4 0.00E+00 0.00E+00 0.0E+00 ;---------------------------------------------------------------