AIPS HELP file for UVBAS in 31DEC24
As of Wed May 8 9:11:21 2024
UVBAS: Averages several channels and subtracts from uv data.
INPUTS
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
BADDISK Disks to avoid for scratch
HELP SECTION
UVBAS
Task: This task does a spectral baseline subtraction 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. More than one
source may be done. Only those sources included by
SOURCE, QUAL, and CALCODE will be in the output data set.
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.
BADDISK....The disk numbers to avoid for scratch files (sorting
tables mostly).
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.