; UVBAS ;--------------------------------------------------------------- ;! averages several channels and subtracts from uv data. ;# Task UV SPECTRAL ;----------------------------------------------------------------------- ;; Copyright (C) 1995 ;; 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 ;----------------------------------------------------------------------- ;--------------------------------------------------------------- UVBAS LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC UVBAS Averages several channels and subtracts from uv 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 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 Lowest channel to write ECHAN Highest channel to write APARM 1,2 & 3,4 channel ranges ---------------------------------------------------------------- UVBAS Task: This task does a spectral baseline subtractraction by averaging two ranges of channels in the input data set and subtracting them from all channels. The two ranges count equally in determining the amount to be subtracted. 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 for the file. BCHAN......Lowest channel number in the input file to write to the output file. 0=> 1. ECHAN......Highest channel number in the input file to write to the output file. 0=> highest in input data. APARM......Channels APARM(1) to APARM(2) and APARM(3) to APARM(4) are to be averaged and subtracted from all channels. ---------------------------------------------------------------- UVBAS: Task which subtracts continuum from channels in UV-plane DOCUMENTOR: H.J. van Langevelde (Sterrewacht Leiden) PURPOSE UVBAS will estimate the continuum visibilities and subtract these from a specified range of channels, hopefully leaving only the information about spectral features in the output UV-file. In the channel ranges APARM(1)-APARM(2) and APARM(3)-APARM(4) the data is averaged. Both complex numbers are then averaged to get a first order accurate estimation for the continuum visibility in the midpoint between these ranges. This value is subtracted from the channels specified in BCHAN - ECHAN, performing what in single dish is known as frequency switching or baseline subtraction. This can only work properly if the UV coverage is the same for all spectral line channels. It can, however, deal with frequency dependent flagging. COMMENTS This tasks has proved to be powerful in problems where bandwith is small and extended continuum emission has to be removed. Its accuracy is limited by the fact that the visibilities in the uv plane change over the bandwith. The errors made in the approximation of the continuum visibility --and thus in the subtraction-- can be estimated as: d V d u d V L D V = --- * --- * D v = --- * --- * D v L d u d v d u c Where D V is the error, D v the bandwith over which we try to do this and u a coordinate in UV-plane. L is the baseline length specified in the same units as those used for c, the speed of light. The formula tells us that UVBAS should do an accurate job on short baselines and/or small bandwith, provided there is a reasonably smooth signature in the UV-plane. That means that it will generally not work for a field dominated by discrte (point)sources. In comparison with the method of cleaning the background and UVSUB the components from the spectral line channels, this method has the main advantages that it is 1) much faster, 2) will work when the background is difficult to model with clean components. When point- sources are the main source of continuum emission the UVSUB method is perfectly suited. In some cases a hybrid method may be advantageous. In comparison with averaging maps to estimate the continuum this method is again faster and more reliable, since there will be no sidelobs of the continuum in the map. The output data set can also be a powerful diagnostic tool. In principle, the data can be used to apply selfcal on your spectral line data.