AIPS HELP file for LPCAL in 31DEC22
As of Thu Jun 30 0:35:25 2022
LPCAL: Task to compute instrumental polarization corrections
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 #
Data selection (multisource):
CALSOUR Source to calibrate with
TIMERANG Time range to use.
SELBAND Bandwidth to select (kHz)
SELFREQ Frequency to select (MHz)
FREQID Freq. ID to select.
BCHAN 0.0 2048.0 Start channel 0=>all
ECHAN 0.0 2048.0 End channel 0=>all
BIF 0.0 100.0 Lowest IF number 0=>all
EIF 0.0 100.0 Highest IF number 0=>all
ANTENNAS Antennas to solve for.
UVRANGE 0.0 UV range in kilolamdba
SUBARRAY 0.0 1000.0 Subarray, 0=>1
Cal. info for input:
DOCALIB -1.0 101.0 > 0 calibrate data & weights
> 99 do NOT calibrate weights
GAINUSE CAL table to apply.
CLEAN map. If not given,
unpol. source is assumed.
BLVER BL table to apply.
FLAGVER Flag table version
DOBAND -1.0 10.0 If >0 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.
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 #
IN2VERS -1.0 255.0 First CC file version #.
NCOMP Number of sub-models,
1 value per field
NMAPS 0.0 4096.0 No. Clean map files (fields)
CMETHOD Modeling method:
CMODEL Model type: 'COMP','IMAG'
'SUBI' (see HELP re images)
SOLINT Preavg. interval (min)
0 => no averaging.
PRTLEV -1.0 10.0 Print statistics 0=>results
CPARM Task enrichment parameters
(1) > 0 => average in IF
and find common solution.
(2) > 0 => compute model
using the image
BADDISK 0.0 1000.0 Disk no. not to use for
Task: This task reads a data UV file and a total intensity source
model and determines the effective feed parameters for each
antenna and IF. These parameters are placed in the antenna
(AN) table. Polarization corrections can then be applied by
setting DOPOL=1 in LISTR, VPLOT, POSSM, SPLIT, IMAGR, and many
Model images made with both values of IMAGR's DO3DIMAG
option are handled correctly, as are multi-scale images. Set
NMAPS = NFIELD * NGAUSS.
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.
The following are used for multisource data files only:
CALSOUR....Source to use for feed calibration. Only one can
and will be used. Must be set for multisource
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.
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.
BCHAN......First channel to use. 0=>1
ECHAN......Highest channel to use. 0=>all higher than BCHAN
BIF........First IF to process. Old values for feed parameters
and calibrator polarizations for unprocessed IFs
are unchanged. 0=>all.
EIF........Highest IF to process. 0=>all higher than BIF
ANTENNAS...A list of the antennas to have solutions
determined. If any number is negative then all
antennas listed are NOT to be used to determine
solutions and all others are. All 0 => use all.
The following may be used for all data files (except as noted):
UVRANGE....Range of projected spacings to be included in
1000's of wavelengths. 0 => 1, 1.E10
SUBARRAY...Subarray number to use. 0=>1.
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.
BLVER......Version number of the baseline based calibration
(BL) table to appply. <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
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
IN2NAME....Cleaned map name (name). If not given, LPCAL assumes that
the source is unpolarized. Note: a CLEAN image may be
given only for one source. An Ipol, Lpol or Rpol image is
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.
IN2CLASS...Cleaned map name (class).
IN2SEQ.....Cleaned map name (seq. #). This must be the
same for all fields.
IN2DISK....Disk drive # of cleaned map. 0 => any.
IN2VERS....CC file version # for the first sub-model.
Applies to all fields. 0 => 1.
NCOMP......THIS ADVERB IS USED IN A NON-STANDARD WAY IN LPCAL:
Number of regions within the facet, each supported by a
separate well-prepared Clean components file, to be
used. The CC file version numbers to be used are IN2VERS
through IN2VERS+NCOMP(i)-1 for sub-models 1 through
NCOMP(i) in facet i. There is a limit of 10 sub-models
over all facets included. The polarization of each
sub-model may be different and will be determined by
LPCAL along with the instrumental parameters. Each
sub-model CC file should be prepared with CCEDT or TABED
or in the imaging to include only the region over which
the source polarization is thought to be roughly
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
' ' allows the program to use the fastest
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.
SOLINT.....Time interval to average data before determining
correction in min. 0 => no averaging.
PRTLEV.....If larger than 0.0, show fit residuals.
CPARM......Task enrichment parameters:
(1) If CPARM(1) is greater than 0 then data in the
selected IFs will be averaged before doing the
solutions. NOTE: the IFs must be phase
coherent for this to be useful.
(2) If CPARM(2)>0 will compute models from images rather
than CLEAN components. But you must also set CMETHOD
BADDISK....Disk numbers on which scratch files are not to
LPCAL: Task to determine effective feed polarization
parameters for VLBI arrays.
Documentor: K. J. Leppanen
Related Programs: CALIB, LISTR, SPLIT, CLCOR, CCEDT,PCAL,SPCAL
PCAL, SPCAL or LPCAL?
PCAL was designed for the VLA and works well for unresolved
of slightly resolved calibrators with uniform polarization.
LPCAL was designed for VLBI and allows the use of calibrators
that have significant structure and differential polarization
across the source. It will also handle unpolarized calibrators.
SPCAL was designed for spectral line experiments and solves
for frequency dependent D-terms. Like PCAL SPCAL demands
an unresolved calibrator.
Polarization calibration of synthesis array visibility
data consists of two distinct parts: 1) the determination of
the effective response of the feeds to the incident radiation
and the correction of the observations to the values which
would have been obtained with perfect feeds and 2) the
determination and removal of systematic phase delay
differences between the right and left hand polarization
systems. LPCAL determines the effective response of the feeds
and stores this information in the AN table. Routines which
can apply calibration tables can then be instructed to apply
the polarization corrections by setting the adverb DOPOL=TRUE.
LPCAL can be used in two cases: 1) the polarization
structure of the calibrator is not known, but a total
intensity model for it is available and can be used to derive
the polarization structure in the way explained below; 2) the
calibrator is unpolarized. In the first case the parallactic
angle range for the calibrator has to be sufficient to allow
for the solution of source polarization. For a bright
unpolarized calibrator even one scan should be sufficient and
no total intensity model is required. LPCAL uses a linear
approximation for the feeds, which becomes inaccurate if the
polarization leakage in the feeds (D-terms) exceeds 5
percent. This should not be the case for new instruments like
the VLBA. Note that the parallactic angles of the source must
differ at different interferometer elements. This means that
VLA data cannot be calibrated with LPCAL.
A source with unknown polarization structure can be used
as a calibrator if its polarization structure can be modeled
as a weighted sum of a small number of sub-models, derived by
simply splitting the total intensity (Stokes I) CLEAN
model. The weights in the sum are initially unknown complex
values that are solved for during the feed solution. Such a
model is likely to be a good representation of the
polarization structure of maser sources, which consist of
distinct masing regions. It is often an adequate assumption
for continuum sources that can be well modeled by a small
number (say, less than five) of gaussians.
STEPS BEFORE RUNNING LPCAL
Before fringe fitting, apply the correction for
parallactic angle using CLCOR, OPCODE='PANG'. Fringe fit the
parallel hand visibilities using normal procedures for
multiband data making sure that all solutions are referenced
to the same antenna. Don't let FRING rereference the solutions
if a change in the reference antenna cannot be avoided; i.e.
set DPARM(7) = 1. Instead, use SNSMO to rereference the
solutions. To find R-L phase and single-band delay offsets
between the IFs, use BLAVG, FRING, and POLSN as described in
the BLAVG help file. Self-calibrate the parallel hand data;
this will also yield a total intensity source model for LPCAL.
If the systematic phase delay difference between the
right and left hand systems is time variable (e.g. variable
ionospheric Faraday rotation at the reference station used in
fringe fitting) then the phase drift needs to be removed
before running LPCAL (see CLCOR). If the systematic phase
offsets are essentially constant then they may be removed
after running LPCAL. To do this, first use LISTR with
STOKES='POLC'; DOPOL=1 and appropriate calibration on a source
with known polarization angle to determine the phase offset
and then apply a correction using CLCOR and OPCODE='POLR'.
Note: this will modify the AN table (rotate the D-term phases)
as well as the CL table.
TOTAL INTENSITY MODEL
For a calibrator with unknown polarization, LPCAL needs a
total intensity source model, which is usually given as a set
of sub-models (CC tables). The division into sub-models can be
made with the task CCEDT automatically or manually. The
division should be made in such a way that the polarized
structure of the resulting sub-models is well approximated by
their (scaled) total intensity structure with constant
polarization angle (similarity assumption). Usually one would
use a Stokes I model, but Stokes L or R models are also
accepted. It is crucial that the sum of the total intensity
sub-models gives a good fit to the parallel-hand
visibilities. Otherwise there is no hope that it can be the
basis for a successful model of the polarization structure.
In case of H2O or SiO masers, each significant maser spot
should constitute an independent sub-model, since each can
have a different degree and angle of linear polarization.
These models can be given as separate CC tables, obtained by
splitting the original CC table with CCEDT. If the source has
very extended structure it may be necessary to map it using
multiple fields in IMAGR. LPCAL can read CLEAN models from
multiple fields, each having one or more CC tables (again, one
per sub-model). Give the number of fields in NMAPS, the
CC version number of the first model in IN2VERS (applies to
all fields), and the numbers of consecutive CC models to use
in NCOMP (one value per field).
If using a continuum calibrator, the preferable way to
produce the sub-models may be the automatic splitting feature
of CCEDT. Try running CCEDT with CPARM=10,0 on the CC table
produced by IMAGR. For manual splitting with CCEDT (and TVBOX, for
example), it may be useful to make a heavily super-resolved image to
see how the CLEAN components tend to group.
IN2NAME, IN2CLASS, IN2SEQ, IN2DISK:
These specify the total intensity model to use. A total
intensity model is irrelevant if emission from all parts of
the source is unpolarized, in which case these should be
left empty. Note, however, that even then the input uv-data
must have been self-calibrated, i.e., the complex gains must
IN2VERS, NCOMP, NMAPS:
See example 1 above.
The data will be preaveraged to SOLINT minutes before the
solution. This should be set sufficiently short to avoid
smearing of source structure. For extended maser sources 0.5
minutes can be too much! Regardless of SOLINT, only one
solution per station and possibly IF is made by LPCAL.
LPCAL will solve for the fractional linear polarization
of each sub-model and print out the solutions. PRTLEV<0
suppresses this printing. Note that the source solutions are
not saved anywhere. PRTLEV=1 (recommended) produces useful
information about the residuals.
SINGLE SOURCE FILES
In principle, LPCAL can work on a single source file
followed by SPLIT to apply the correction. However, in
practice, it is preferable to convert the input file to
multisource file using task MULTI followed by INDXR to create
an index table.