AIPS HELP file for UVLSF in 31DEC24
As of Fri Apr 26 18:28:09 2024
UVLSF: Least squares fit baseline 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 #
SRCNAME 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 -4.0 4.0 Order of fit line (0 -> DC)
< 0 => add contiuum of
CHANNEL back to line data
DOOUTPUT -1.0 1.0 > 0 => write fit baseline as
a continuum uv data base
CHANNEL -16384.0 1634.0 Channel of fit to be written
as continuum: 0 -> reference
FLUX 0.0 Flag data if residual flux in
any fit channel is > FLUX
0 -> 10**20
CUTOFF 0.0 Flag data if RMS of residual
in fit channels > CUTOFF
0 -> 10**20
SHIFT Shift in asec at ref position
FQCENTER >= 0 -> center frequency axis
BADDISK Disks to avoid for scratch
HELP SECTION
UVLSF
Task: This task does a spectral baseline subtraction. It fits a
straight line to the real and imaginary parts of selected
channels and subtracts the fitted baseline from the spectrum.
Optionally, it can flag data having excess residuals in the
channels used for the baseline fitting. This is a good method
for detecting and deleting data damaged by RFI.
See task FLGIT for a more powerful RFI excision task and UVMLN
for a task dealing with multi-source data with FG tables.
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.
SRCNAME....Source name to be gridded. Must specify if input is
a multi-source data set, otherwise all sources are
included.
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.
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 applying to 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.
**********************************************************
Experimentally, for special transition problems with the
VLA, ORDER is allowed to go up to 4. If ORDER = -n is
entered, then the fit is done with order n and the fit
value at channel CHANNEL is added back to all of the line
channels. This leaves one with a data set containing
continuum and line signal, but with channel-dependent
problems in the continuum suppressed.
**********************************************************
DOOUTPUT...<= 0 => write out only the subtracted uv data.
> 0 => write also a data set with the fit baseline
as visibilities.
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. <= 0 => 1.0E20
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. <= 0 => 1.0E20
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
tables mostly).
EXPLAIN SECTION
UVLSF: Task which subtracts continuum from channels in UV-plane
DOCUMENTOR: H.J. van Langevelde (Sterrewacht Leiden)
RELATED TASKS: UVLIN, UVBAS
PURPOSE
UVLSF 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.
PHASE SHIFTING
Since UVLSF 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.
COMMENTS
This task 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 UVLSF 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.
UVLSF 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 weight
before deciding on flagging. This is correct for peak residuals due
solely to thermal noise (and reflected correctly in the data weights),
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 integration time typical for the current data set). The
CUTOFF test is better for spectra that do not fit the linear model or
are just excessively noisy for some reason. UVLSF 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.
REFERENCES
See Cornwell, Uson and Haddad (Astron. Astrophys. 258, 583; 1992) for a
detailed discussion which includes estimates of errors.