; UVBAS ;--------------------------------------------------------------- ;! averages several channels and subtracts from uv data. ;# Task UV SPECTRAL ;----------------------------------------------------------------------- ;; Copyright (C) 1995, 2008, 2010 ;; 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 # SOURCES Source name QUAL -10.0 Calibrator qualifier -1=>all CALCODE Calibrator code ' '=>all TIMERANG Time range to use SELBAND Bandwidth to select (kHz) SELFREQ Frequency to select (MHz) FREQID Freq. ID to select. SUBARRAY 0.0 1000.0 Sub-array, 0=>all BIF Low IF number to do EIF Highest IF number to do DOCALIB -1.0 101.0 > 0 calibrate data & weights > 99 do NOT calibrate weights GAINUSE CL (or SN) table to apply DOPOL -1.0 10.0 If >0.5 correct polarization. PDVER PD table to apply (DOPOL>0) BLVER BL table to apply. FLAGVER Flag table version DOBAND -1.0 10.0 If >0.5 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. DOACOR Include autocorrelations? 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. SOURCES....Source to be baselined. ' '=> all; if any starts with a '-' then all except ANY source named. Only one source may be done at a time. QUAL.......Qualifier of source to be baselined. -1 => all. CALCODE....Calibrator code of sources to baseline. ' '=> all. TIMERANG...Time range of the data to be copied. 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. SUBARRAY...Sub-array number to copy. 0=>all. BIF........First IF to include. 0 -> 1. EIF........Last IF to include. 0 -> max. 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 table to apply to multi-source files or the SN table for single source files. 0 => highest. DOPOL......If > 0 then correct data for instrumental polarization as represented in the AN or PD table. This correction is only useful if PCAL has been run or feed polarization parameters have been otherwise obtained. See HELP DOPOL for available correction modes: 1 is normal, 2 and 3 are for VLBI. 1-3 use a PD table if available; 6, 7, 8 are the same but use the AN (continuum solution) even if a PD table is present. PDVER......PD table to apply if PCAL was run with SPECTRAL true and 0 < DOPOL < 6. <= 0 => highest. BLVER......Version number of the baseline based calibration (BL) table to apply. <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. IMAGR uses DOBAND as the nearest integer; 0.1 is therefore "false". BPVER......Specifies the version of the BP table to be applied 0 => highest numbered table. <0 => no bandpass correction to be applied. 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). DOACOR.....> 0 => include autocorrelations as well as cross correlation data. 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. The average in each group of channels is done as a vector average. The result is converted to amplitude and phase and that is subtracted from the data. ;----------------------------------------------------------------------- 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.