; FIXWT ;--------------------------------------------------------------- ;! Modify weights to reflect amplitude scatter of data ;# TASK UV CALIBRATION OOP ;----------------------------------------------------------------------- ;; Copyright (C) 1995-1996, 1999, 2003, 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 ;----------------------------------------------------------------------- FIXWT LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC FIXWT: Modify weights to reflect amplitude scatter of data. 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 drive # 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 #. DOUVCOMP Write compressed data? FLAGVER Flag table to apply. BCHAN 0.0 4096.0 Begin channel 0 => 1 ECHAN 0.0 End channel SOLINT 0.0 Averaging interval DOALL -1.0 1.0 > 0 -> find overall scaling factor for weights CPARM Limit controls BADDISK Disk no. not to use for scratch files. ---------------------------------------------------------------- FIXWT Task: AIPS assumes that the weights given to UV data points are the inverse square of the estimated standard deviation of the amplitudes due to thermal (and other) noise. Unfortunately, not all input data comforms to this requirement. FIXWT runs a filter of width SOLINT minutes over the data and calculates new weights for each antenna as function of time from the scatter of the visbilities. The data must be in time order, but the baselines at any given time may be in any order. Subarray ID is ignored, so multiple subarrays may exist only if they are separated in time. Multiple sources or Frequency IDs cause an integration to break. 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 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. Note that the input file must be TB sorted 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 w room DOUVCOMP...If DOUVCOMP > 0 then the output data (and the scratch files) will be compressed. FLAGVER....specifies the version of the flagging table to be applied. 0 => highest numbered table. <0 => no flagging to be applied. Flags are applied when reading data to determine weights but are not applied when reading the data to merge the weights in with the visibilities. BCHAN......Begin channel number. 0 -> 1 ECHAN......End channel number. 0 -> highest The channels are separate samples of the amplitude; all channels will be written to the output. SOLINT.....Averaging interval (minutes). 0 -> 1. You must have at least 5 samples to compute an rms - these may be multiple spectral channels or multiple times. DOALL......IF <= 0, the weights are found by antenna for each SOLINT (broken if source or FQ changes) and applied in that form. If > 0, the weights are found in the same way, but are then compared to the existing weights to determine an over-all correction factor to be applied to all weights. CPARM......Values can get out of hand here so limits things as (1) Minimum rms allowed (Jy): 0 -> 0. (2) Maximum rms allowed (Jy): 0 -> very large (3) Minimum antenna weight allowed: 0 -> 0 (4) Maximum antenna weight allowed: 0 -> very large Note a baseline weight is a product of 2 antenna weights and should be 1 / sigma**2. (5) When DOALL is false, the weights are found from the rms averaging over, in principle, multiple records. These records may have different weights in averaged data, representing different integration times. If CPARM(5) <= 0, then the output weights will be 1/rms**2 ignoring differences in integration time. You may have the task down-weight samples that have lower weights, setting the actual weight to (WTout) * (WTin/Max(WTin))**CPARM(5) where WTout is the weight based on the rms, WTin is the individual sample's weight on input and Max(WTin) is the maximum of WTin over the whole data set. If the input weights are of the "old" type representing a simple count of 10-second records, set CPARM(5)=1. If the input data are averaged but with weights representing an estimate of system temperature, set CPARM(5) to something between 0 and 1, e.g. 1/3. BADDISK....Disk numbers on which scratch files are not to be placed. ---------------------------------------------------------------- FIXWT: Task to fix uv data weights Documentor: Chris Flatters Related Programs: almost all programs operating on UV data AIPS assumes that the weight given to a uv data point is the statistically correct weight of unity divided by the estimated variance due to noise. Various data input sources, including the VLA and the VLBA, do not always follow this convention, however. This can lead to a number of problems ranging from the relatively trivial problem of incorrectly plotted error bars in uv plots to more serious problems involving calibration and imaging, particularly where data from more than one array are being combined. FIXWT deals with this problem by copying its input file, assigning new weights, based on the actual standard deviations of the visibilities in the file. It assumes that the same weights may be assigned to all spectral channels, but sets different weights for IFs and polarizations. The weights are determined independently for each SOLINT integration time on the assumption that the rms squared on a given baseline is the product of the separate antenna rms's. The weights are not assumed to be continuous over changes of source or frequency ID or between SOLINT intervals. Data points will be flagged by setting their weights to zero in ranges where the standard deviation of the data cannot be determined due to insufficient data. Increasing SOLINT will give a smoother estimate of the noise. Fully flagged visibilities will not appear in the output file so INDXR should be run on the output file if the input was a multi-source file. This step can be omitted, however, if the number of visibilities in the output file is the same as that in the output file.