AIPS HELP file for UVLSD in 31DEC22
As of Mon Sep 25 23:08:35 2023
UVLSD: Least squares fit baseline and divides the 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
ICHANSEL Select channels to fit: NOTE
this is start,end,increment
and IF for each region
ORDER 0.0 1.0 Order of fit line (0 -> DC)
DOOUTPUT -1.0 1.0 > 0 => write fit baseline as
a continuum uv data base
CHANNEL 0.0 9999.0 Channel of fit to be written
as continuum: 0 -> reference
FLUX 0.0 Flag data if residual flux in
any fit channel is > FLUX
CUTOFF 0.0 Flag data if RMS of residual
in fit channels > CUTOFF
SHIFT Shift in asec at ref position
FQCENTER >= 0 -> center frequency axis
BADDISK Disks to avoid for scratch
Task: This task does a spectral baseline division. It fits a
straight line to the real and imaginary parts of selected
channels and divides the fitted baseline into the
spectrum. UVLSF does the normal operation of subtracting the
continuum - this task is for situations in which one is looking
for weak absorption. By dividing by the continuum, the
continuum source structure is eliminated. If the line signal
is uniformly distributed over the continuum, this operation
will maximize the resulting apparent line signal for averaging
Optionally, it can flag data having excess residuals in the
chanels used for the baseline fitting. This is a good method
for detecting and deleting data damaged by RFI.
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
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
(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
IMAGR uses DOBAND as the nearest integer; 0.1 is therefore
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
DOACOR.....> 0 => include autocorrelations as well as cross
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.
ICHANSEL...Select up to 20 groups of channels/IF(s) to fit as sets
of (Start,end,inc,IF), i.e., ICHANSEL = 6,37,1,0,
92,123,1,0 for two regions applyingto all IFs. The first
group for which ICHANSEL(2,i) <= 0 ends the list.
Defaults: Any IF having no group assigned to it, gets a
group including all channels.
ICHANSEL(1,j) defaults to 1,
0 < ICHANSEL(2,j) < ICHANSEL(1,j) defaults to Nchan.
ICHANSEL(3,j) < 1 or > ICHANSEL(2,j)-ICHANSEL(1,j)+1
defaults to 1.
ICHANSEL(4) <= 0 => this group applies to all IFs.
ORDER......Normally the fit should be done with a first order
polynomial, namely a DC term and a slope. If only one box
is used, the slope may not be adequately defined and a
simple DC term for the real and for the imaginary parts
would be more reliable.
DOOUTPUT...<= 0 => write out only the subtracted uv data.
> 0 => write also a data set with the fit baseline
CHANNEL....When DOOUTPUT > 0, write the continuum data with
the fit evaluated at spectral channel CHANNEL.
0 => reference channel.
FLUX.......If the residual flux in any channel used to fit the
baseline exceeds FLUX, then the spectrum for that
time, IF, and polarization is fully flagged.
CUTOFF.....If the RMS flux in the channels used to fit the
baseline exceeds CUTOFF, then the spectrum for that
time, IF, and polarization is fully flagged.
SHIFT......Shift in arcseconds during fitting process. The data
are shifted, fitted, flagged and then shifted back.
These are shifts in arc seconds at the reference point -
RA = RA0 + SHIFT(1)/cos(DEC0), DEC = DEC0 + SHIFT(2)
FQCENTER...> 0 => Change frequency axis reference pixel to
Nchan / 2 + 1
else => do not change reference pixel
BADDISK....The disk numbers to avoid for scratch files (sorting
UVLSD: Task which subtracts continuum from channels in UV-plane
DOCUMENTOR: H.J. van Langevelde (Sterrewacht Leiden)
RELATED TASKS: UVLIN, UVBAS
UVLSD 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.
It fits, by least squares, a straight line to the to the real and
imaginary parts of the selected channels. This baseline is
subtracted from the spectrum.
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.
Since UVLSD works perfectly for a single point source at the
phase center, one should shift any dominant point source to the phase
center prior to the fitting process. Note that UVFIX now does this
correctly since it does the relative phase shift between channels.
The shift option is offered here for convenience and since THE SHIFT
IS UNDONE AFTER THE FIT. The sense of the shift is the same as in
UVFIX. The code for computing the shift is:
DTORAD = 3.14159265358979326D0 / 180.0D0
RA = RA0 + SHIFT(1) / 3600./ COS (DTORAD * DEC0)
DEC = DEC0 + SHIFT(2) / 3600.
DXC = SIN (DTORAD * (RA - RA0)) * COS (DEC * DTORAD)
DYC = COS (DEC0 * DTORAD) * SIN (DEC * DTORAD) -
* SIN (DEC0 * DTORAD) * COS (DEC * DTORAD) *
* COS ((RA - RA0) * DTORAD)
DZC = SIN (DTORAD * DEC0) * SIN (DTORAD * DEC) +
* COS (DTORAD * DEC0) * COS (DTORAD * DEC) +
* COS (DTORAD * (RA - RA0)) - 1.0D0
The necessary values for SHIFT can be measured from an image in arc
seconds or have position routines return true coordinates which must
be corrected by multiplying by cos(declination) in RA.
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
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 UVLSD 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 discrete
(point) sources. It will work better if the dominant point
source is shifted to the filed center. Use UVFIX which now does
a correct shift for spectral data.
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.
UVLSD differs from UVLIN in a number of ways. UVLIN does its
flagging only on the peak residual in the channels selected for
fitting and scales the peak by the square root of the
integration time before deciding on flagging. This is correct
for peak residuals due solely to thermal noise but is an added
confusion to the user. In any case, you should set the FLUX
cutoff at a conservative level (6 - 8 times the expected sigma
in the usual integration time). The CUTOFF test is better for
spectra that do not fit the linear model or are just excessively
noisy for some reason. UVLSD offers the option of writing the
fit continuum as a UV data set which can be used for self-cal,
TVFLG editing, etc.
UVBAS fits the linear baseline in amplitude and phase
rather than real and imaginary. If the emission is well off the
center of the field, the amplitude and phase are more likely to
be nearly linear than the real and imaginary parts. However,
the amplitude does not have Gaussian noise statistics and UVBAS
is therefore suspect in low signal-to-noise ratio cases.
See Cornwell, Uson and Haddad (Astron. Astrophys. 258, 583; 1992) for a
detailed discussion which includes estimates of errors.