AIPS HELP file for BLCHN in 31DEC24
As of Sat Apr 20 8:14:54 2024
BLCHN: Task to compute closure offset corrections per channel
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 #
OUTNAME Output UV file name (name)
OUTCLASS Output UV file name (class)
OUTSEQ 0.0 9999.0 Output UV file name (seq. #)
OUTDISK 0.0 9.0 Output UV file disk unit #
Data selection (multisource):
SOURCES Sources to calibrate with
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 Subarray, 0=>all
Cal. info for input:
FLAGVER Flag table version
DOCALIB -1.0 101.0 > 0 calibrate data & weights
> 99 do NOT calibrate weights
GAINUSE CAL table to apply.
CLEAN map (optional)
DOPOL -1.0 10.0 If >0 correct polarization.
PDVER PD table to apply (DOPOL>0)
DOBAND -1.0 10.0 If >0 apply bandpass cal.
Method used depends on value
of DOBAND (see HELP file).
BPVER Bandpass table version
BLVER -1.0 46655.0 INPUT BL table version
SMOOTH Smoothing function. See
HELP SMOOTH for details.
IN2NAME Cleaned map name (name)
IN2CLASS Cleaned map name (class)
IN2SEQ 0.0 9999.0 Cleaned map name (seq. #)
IN2DISK 0.0 9.0 Cleaned map disk unit #
INVERS -1.0 46655.0 CC file version #.
NCOMP # comps to use for model.
1 value per field
FLUX Lowest CC flux included
NMAPS 0.0 4096.0 No. Clean map files
CMETHOD Modeling method:
'DFT','GRID',' '
CMODEL Model type: 'COMP','IMAG'
'SUBI' (see HELP re images)
SMODEL Source model, 1=flux,2=x,3=y
BE CAREFUL overrides IN2NAME
ICHANSEL Array of start and stop chn
numbers, plus a channel
increment and IF to be used
for channel selection in the
averaging. See HELP ICHANSEL.
Default = full band
DOSCALE -1.0 2.0 = -1 -> no spectral index
correction
- 0, 1 -> fit spectral index
= 2 -> fit curvature also
SPECINDX Spectral index to correct
SPECURVE Spectral index curvature
BPARM Control info:
1 > 0 => already divided
2 > 0 => print statistics
3-5 > 0 => smooth the
solution
BADDISK Disk no. not to use for
scratch files.
HELP SECTION
BLCHN
Task: This task reads a UV file, calibrates, divides by a model,
averages the data over all included times on each baseline, and
saves the resulting BL information as a function of frequency.
Known bad channels may have the average over the good channels
substituted for the suspect average. The final averages are
applied to the input file (all sources, no calibration, no
flagging) on a channel-by-channel basis and written to the
output file.
This is an experimental task. The assumption is that the uv
data must have a well-determined antenna-based calibration
before BLCHN is run. For multi-source input files, the antenna
based gain solutions must be applied when running BLCHN; these
solutions and the source models must be such that after the
calibrated data are divided by the model, all data have
amplitudes near 1 and phases near 0. Source models input to
BLCHN must describe ALL of the flux in the calibrators (including
any significant confusing flux). In general, the BL
correction should be a slow function of channel. Therefore,
for improved S/N, you should smooth the input data substantially
with SMOOTH. The IF-dependent, channel-independent solutions by
BLCAL produce a BL table which is understood by the standard
calibrations and can be time-dependent. BLCHN on the other hand
produces a channel-dependent, time-independent solution which
is written into a BD table and applied to the data to produce a
new outfile data set. Task BDAPL may be used thereafter to apply
the BD solution to a different data set. Note that that data set
must have the same number of spectral channels and IFs as the
input data of BLCHN.
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 w room
The following are used for multi-source data files only:
SOURCES....List of sources for which calibration constants
are to be determined. '*' = all; a "-" before a
source name means all except ANY source named.
Note: solutions for multiple sources can only be
made if the sources are point sources at their
assumed phase center and with the flux densities
given in the source (SU) table. All ' ' =>all.
TIMERANG...Time range of the data to be used. 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, 0=> all
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, 0=> all
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 overide that of FREQID.
However, setting SELBAND and SELFREQ may result in
an ambiguity, in which case the task will request
that you use FREQID.
The following may be used for all data files (except as noted):
SUBARRAY...Subarray number to use. 0=>all.
FLAGVER....(multisource) specifies the version of the flagging table
to be applied. 0 => highest numbered table. <0 => no
flagging to be applied.
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 or SN table to apply to
the data. 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.
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.
BPVER......Specifies the version of the BP table to be applied.
<0 => no bandpass correction done.
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).
IN2NAME....Cleaned map name (name). Standard defaults.
Note: a CLEAN image for only a single source may be given
although it may be in a multi-source file.
For a multi-source file, the flux of the clean components
selected for the model are summed and scaled to the source
flux found in the SU table. If that flux is zero, no
scaling is done.
IN2CLASS...Cleaned map name (class). Standard defaults.
IN2SEQ.....Cleaned map name (seq. #). 0 -> highest.
IN2DISK....Disk drive # of cleaned map. 0 => any.
INVERS.....CC file version #. 0=> highest numbered version
NCOMP......Number of Clean components to use for the model, one
value per field. If all values are zero, then all
components in all fields are used. If any value is not
zero, then abs(NCOMP(i)) (or fewer depending on FLUX and
negativity) components are used for field i, even if
NCOMP(i) is zero. If any of the NCOMP is less than 0,
then components are only used in each field i up to
abs(NCOMP(i)), FLUX, or the first negative whichever
comes first. If abs(NCOMP(i)) is greater than the number
of components in field i, the actual number is used. For
example
NCOMP = -1,0
says to use one component from field one unless it is
negative or < FLUX and no components from any other
field. This would usually not be desirable.
NCOMP = -1000000
says to use all components from each field up to the
first negative in that field.
NCOMP = -200 100 23 0 300 5
says to use no more than 200 components from field 1, 100
from field 2, 23 from field 3, 300 from field 5, 5 from
field 6 and none from any other field. Fewer are used if
a negative is encountered or the components go below
FLUX.
FLUX.......Only components > FLUX in absolute value are used in the
model.
NMAPS......Number of image files to use for model. For multi-scale
models, set NMAPS = NFIELD * NGAUSS to include the Clean
components of the extended resolutions. If more than one
file is to be used, the NAME, CLASS, DISK and SEQ of the
subsequent image files will be the same as the first file
except that the LAST 3 or 4 characters of the CLASS will
be an increasing sequence above that in IN2CLASS. Thus,
if INCLASS='ICL005', classes 'ICL005' through 'ICLnnn'
or 'ICnnnn', where nnn = 5 + NMAPS - 1 will be used. Old
names (in which the 4'th character is not a number) are
also supported: the last two characters are '01' through
'E7' for fields 2 through 512. In old names, the highest
field number allowed is 512; in new names it is 4096.
CMETHOD....This determines the method used to compute the
model visibility values.
'DFT' uses the direct Fourier transform, this
method is the most accurate.
'GRID' does a gridded-FFT interpolation model
computation.
' ' allows the program to use the fastest
method.
NOTE: when using a model derived from data with
different uv sampling it is best to use 'DFT'
CMODEL.....This indicates the type of input model; 'COMP' means that
the input model consists of Clean components, 'IMAG'
indicates that the input model consists of images.
'SUBI' means that the model consists of a sub-image of
the original IMAGR output. If CMODEL is ' ' Clean
components will be used if present and the image if not.
SUBI should work for sub-images made with DO3DIM true and
sib-images of the central facet made with DO3DIM false,
but probably will not work well for shifted facets with
DO3DIM false. Use BLANK rather than SUBIM in such cases.
CALIB will set a scaling factor to correct image units
from JY/BEAM to JY/PIXEL for image models. If the source
table contains a flux, then that flux will be used to
scale the components model to obtain the stated total
flux. This is needed since initial Cleans may not obtain
the full flux even though they represent all the
essentials of the source structure.
SMODEL.....A single component model to be used instead of a CLEAN
components model; if abs (SMODEL) > 0 then use of this
model is requested.
SMODEL(1) = flux density (Jy)
SMODEL(2) = X offset in sky (arcsec)
SMODEL(3) = Y offset in sky (arcsec)
SMODEL(4) = Model type:
0 => point model (only model inplemented)
NOTE: If neither a CLEAN nor a point model is given then
a point model using the flux densities given in the
source table is used.
ICHANSEL...Array of start and stop channels plus a channel increment
and IF, used to select the channels thought to be
"good". Channels not included in ICHANSEL have their
(suspect) averages replaced by the average over the
channels included in ICHANSEL. For instance, if you
know that channels 1 - 10 and 121 - 128 are dubious
because of bandpass effects, and channels 56 - 80 of IF 1
but not IF 2 because of interference, then you would set
ICHANSEL = 11,55,1,1, 81,121,1,1, 11,121,1,2. Channels
1-10, 56-80, and 121-128 will have their averages
replaced by the average over channels 11-55 plus 81-120
in IF 1. Channels 1-10 abd 121-128 will have their
averages replace by the average of channels 11-120 in IF
2. Up to 20 groups of start, stop and increment channel
numbers plus IF numbers can be specified. The default
(ICHANSEL = 0) is to call all channels good. If ICHANSEL
describes bad channels explicitly for some IFs, but skips
other IFs, then all channels are taken as good for the
skipped IFs. For example: ICHANSEL=2,6,1,2 => The
channels 2-6 will be used as good for IF=2 and all
channels will be taken as good for the rest of the IFs.
Channels 1 and 7-N will be replaced with the average of
channels 2-6 in IF 2 only.
DOSCALE....Controls whether spectral index corrections are made.
DOSCALE < 0 => do no spectral index correction.
DOSCALE >= 0 => do spectral index correction, using
SPECINDX and SPECURVE for the first calibrator or
using known parameters, or fitting fluxes in the SU
table.
DOSCALE >= 2 => fit spectral index and curvature for
unknown sources. Do this only for well-determined
fluxes over a wide frequency range.
SPECINDX...The fit baseline corrections are corrected for spectral
index. The source flux is assumed to be proportional to
frequency^SPECINDX so the corrections are multiplied by
(freq/f0) ** SPECINDX when SPECINDX is not zero.
If SPECINDX = 0 and the calibrator source is one of
3C286, 3C48, 3C147, 3C138, 3C123, 3C196, or 3C295 (the
standard "known" ones) SPECINDX and SPECURVE will be set
to the known (Perley 20xx) values from SETJY.
If SPECINDX=0 and the source is not one of the known
ones, then a spectral index (with curvature if DOSCALE=2)
is fit to the fluxes in the source table.
SPECURVE...If SPECINDX is not zero, a curvature term may be added to
the above correction as
A = log(f/f0) * SPECINDX + log(f/f0)^2 * SPECURVE(1)
+ log(f/f0)^3 * SPECURV(2) + log(f/f0)^3 * SPECURV(2)
and then multiply by 10^(-A/2.) where the 2 is because
the BP tables are voltages rather than powers. f0 is
taken to be 1.0 GHz by convention and all logs are base
10.0.
BPARM......Control information:
BPARM(1)...If > 0 then the data has already been divided by the model.
No division by the model will be done. Note that this
option will write out the already divided data set
corrected by the multiplicative correction. In this case,
the output BD table and task BDAPL are probably the most
useful.
BPARM(2)...If this value is larger than 0.0, then the correlator
averages and their RMSs will be printed on the monitor
terminal and the message file.
BPARM(3-5)....If > 0, smooth the solution before it is written to the
BD table and applied to the input data set. BPARM (3)-(5)
have the same meaning as SMOOTH (1)-(3) above.
BADDISK....Disk numbers on which scratch files are not to be placed.
EXPLAIN SECTION
BLCHN: Task to collect baseline-based (closure) corrections.
DOCUMENTOR: R.C.Walker (modified by W.Cotton & A.Bridle)
RELATED PROGRAMS: CALIB, UVSUB, TACOP, SPLIT
PURPOSE
Most modern interferometers are calibrated under the assumption
that the gains needed to calibrate each baseline are simply the
geometric means of the gains of the two antennas involved in the
baseline, i.e. that a single gain can be determined for each antenna
and can then be used to calibrate all baselines to that antenna. This
assumption can break down at levels involved when large dynamic ranges
are required. Possible reasons for the breakdown are mis-matched
bandpasses, pointing errors on large sources, offsets in the
correlator, large delay errors, etc. Errors introduced by such effects
are often called "closure errors" because they invalidate the
assumption that phase and amplitude errors are antenna based and
therefore "close" around some loop of baselines.
The old VLA often had closure errors between 0.5 and 1 percent
and similar, somewhat smaller, errors are present in the EVLA.
These errors vary in time in ways that are not fully understood, but
they are sufficiently constant that measurements of closure offsets
made within a few hours of observations can be used to improve high
dynamic range maps significantly (see Walker, VLA Scientific Memo
152). If the dynamic range (measured as the ratio of peak to
off-source rms) is less than 10,000, closure offsets are probably not
causing problems. If the dynamic range is near 10,000:1 and the map
is not noise-limited, closure offsets may be the limiting effect.
Correction of closure offsets may always be needed to obtain dynamic
ranges >10,000:1. With closure offset corrections, dynamic ranges
>10,000:1 have been achieved (see Lecture 11 in the Course Notes of
the NRAO Summer School on "Synthesis Imaging", held on August 5-9
1985).
Closure offsets are best measured by observing a strong, compact
source. The observing time and source strength should be such that
the data on each baseline has a SNR of nearly 1000. The data from the
compact source should be accurately calibrated before running BLCHN;
the data will be divided by the specified model before averaging. If
multiple compact sources are used for calibration in a given run of
BLCHN, they should be pointlike to high accuracy.
VLA data provided to BLCHN should have amplitudes within a few
percent of 1.0 and phases within a degree or two of 0 after division
by the model; deviations may be worse for other (especially VLBI)
arrays. The data should be examined carefully using LISTR or PRTUV
before running BLCHN.
Once a properly calibrated and edited data set is available,
BLCHN can be run to average the selected data from the input data set
and then apply that correction to all of the data in that data set.
This is done to apply a channel-dependent BL solution rather than the
channel independent solution determined by BLCAL.
BLCHN calculates means and rms's for the baseline corrections and
writes the results in the form of amplitudes and phases in the message
file if requested.
Division of the data by the model may be done in one of four
ways. 1) External to BLCHN, by e.g. OOSUB, this is indicated by
BPARM(1)=1. 2) Division by a CLEAN model, this is indicated by
filling in IN2NAME etc. 3) Specifying a point model, this is
indicated by the appropriate values in SMODEL. 4) Using a point
source at the phase center with the flux density given in the source
(SU) table. This method is used when either none of the above methods
are requested or multiple sources are requested.