AIPS HELP file for PCAL in 31DEC25
As of Wed Dec 11 6:54:39 2024
PCAL: Task to compute polarization corrections
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 #
Data selection (multisource):
CALSOUR Sources to calibrate with
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.
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=>all
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 (optional)
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.
ICHANSEL Array of start and stop chan
numbers, plus a channel
increment and IF to be used
to select channels to sum to
find the polarization.
0 => all channels
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 component used.
NMAPS 0.0 4096.0 No. Clean map files
CMETHOD Modeling method:
'DFT','GRID',' '
CMODEL Model type: 'COMP','IMAG'
'SUBI' (see HELP re images)
DOMODEL -1.0 2.0 > 0 => use model, do not
fit source Q and U
= 2 use CP table model
PMODEL Source poln. model
SPECPARM Spectral index: I Q U V for
each CALSOUR
DOSCALE -1.0 2.0 >= 0 use spectral index
= 2 solve for curvature
SOLINT Soln. interval (min) 0=>5.
SOLTYPE Solution type:
'ORI-', 'APPR', 'RAPR'
SPECTRAL -1.0 1.0 > 0 do spectral PCAL
<= 0 do continuum PCAL
INTPARM Smoothing parameters for
spectral mode
PRTLEV 0.0 10.0 Print statistics 0=>none
1 = some, 2 = lots. Use 1.
REFANT 0.0 90.0 Reference antenna, 0->pick
BPARM Task enrichment parameters
for SOLTYPE 'ORI-' only:
(1) if > 0 use default
initial feed parameters.
(2) > 0 -> no error calc.
(3) if > 0 then fit for R-L
phase difference
(4) initial R-L phase
(5) >0 solve for Vpol.
(6) >0 fix ref. ori. 1
(7) >0 fix ref. ori. 2
(8) >0 fix all orientations
(9) >0 fix all ellipticities
(10) >0 fix source poln.
CPARM Task enrichment parameters
(1) >0 => average in IF
and find common solution.
(2) >0 => update source
table with pol. solution
(3) >0 => do NOT interpolate
over flagged channels
(4) CP table version if
DOMODEL = 2
(5) >0 Use linear model for
linear polarization data
(7) >0 => use initial guess
from AN or PD tables
(8) >0 => max. no. iter
(9) >0 => conv. tolerance
(10) >0 => conv. tol.
DPARM (1) > 0 => take blanked
Faraday rotation as 0
BADDISK 0.0 9.0 Disk no. not to use for
scratch files.
HELP SECTION
PCAL
Task: This task reads a UV file, calibrates, subtracts a model and
determines the effective feed parameters for each antenna and
IF. These parameters are then placed in the antenna (AN)
table. Polarization corrections can then be applied by setting
DOPOL=1 in LISTR or SPLIT.
If a polarized model is given then only the feed
parameters are determined. If no model is given then a point
source is assumed for SOLTYPE='APPR'. For SOLTYPEs 'ORI-' and
'RAPR' the source may be resolved but the polarized flux is
assumed to have the same distribution as the total intensity
and the polarization angle is assumed to be constant.
Model images made with both values of IMAGR's DO3DIMAG
option are handled correctly, as are multi-scale images. Set
NMAPS = NFIELD * NGAUSS.
The frequencies within each IF are averaged by PCAL before
doing the solutions. So the phase has to be flat within each
IF. Therefore the data have to be phase calibrated by FRING or
by PCCOR before PCAL.
In 31DEC10, PCAL has two modes. In the old, familiar one, PCAL
averages spectral channels and determines antenna D terms and
optionally source Q and U on a per IF basis. The results are
written to the antenna (AN) and optionally the source (SU)
tables. In the new mode, PCAL determines the D terms and
optionally the source Q and U on a per spectral channel basis.
Such results are written to the PD and CP tables, respectively.
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.
The following are used for multisource data files only:
CALSOUR....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.
No more than 50 calibrators may be selected using this
and the following two adverbs.
QUAL.......Only sources with a source qualifier number in the SU
table matching QUAL will be used if QUAL is not -1.
CALCODE....Calibrators may be selected on the basis of the
calibrator code:
' ' => any calibrator code selected
'* ' => any non blank code (cal. only)
'-CAL' => blank codes only (no calibrators)
anything else = calibrator code to select.
NB: The CALCODE test is applied in addition to the
other tests, i.e. CALSOUR and QUAL, in the
selection of sources for which to determine
solutions.
The following may be used for all data files (except as noted):
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. 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.
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.
UVRANGE....Range of projected spacings to be included in
1000's of wavelengths. 0 => 1, 1.E10
SUBARRAY...Subarray number to use. 0=>all.
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
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).
ICHANSEL.. Array of start, stop, and increment channel numbers plus
an IF used for channel selection in the averaging to
compute the average values used for solving for
polarization. Up to 20 sets if channels/IF may be
entered. The first having ICHANSEL(2,i) <= 0 terminates
the list. ICHANSEL(4,i) is the IF number, with <= 0
meaning all IFs. If an IF has no ICHANSEL set for it,
then all channels are used.
For instance, if you wished to exclude channels 1 - 10
and 121 - 128 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. If you only wished to use every other
channel from the second IF then you would set ICHANSEL =
11,55,1,1, 81,121,1,1, 11,121,2,2. if there are actually
4 IFs, then IFs 3 and 4 would use channels 17 through
112. To set the channel range for all IFs to 14, 115
enter ICHANSEL = 14,125. To set the channel range for
all IFs to 14-115 except IF 3 set ICHANSEL=14,115,1,0,
23 45,1,3, 64,101,1,3 where the all IF part must come
before the parts that partially override it.
IN2NAME....Cleaned map name (name). Standard defaults.
Read DOMODEL (below) first.
Note: a CLEAN image for only a single source may be given
although it may be in a multi-source file. An Ipol, Qpol,
Upol, and Vpol image of the same name are expected. If
the first one of the polarization is missing, that
polarization is taken to have a value of 0.0. 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. NOTE: if a model
image is given, then the task will not fit for source
polarization if models for at least I, Q, and U are
given. IN2NAME and IN2CLASS cause PMODEL to be ignored.
IN2CLASS...Cleaned map name (class). The value given should be for
the Ipol image, the Q, U, and V pol images will be
assumed to be the same except for an initial Q, U, or V
in the class.
IN2SEQ.....Cleaned map name (seq. #). These should be the same for
I, Q, U, and V images.
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.
DOMODEL....<= 0 => fit for the calibration source(s) Q and U. This
still requires knowledge of the source(s) I flux. If
there is only one source, this is given by PMODEL(1)
if it is greater than 0 (required for single-source
files) with a default of the fluxes in the source
table for multi-source files. An image model is also
possible for a single source. If there is more than
one calibration source, then PMODEL and IN2NAME et al.
are ignored and the I fluxes come from the source
table.
> 0 => use a model for the calibration source(s) I, Q,
and U (and maybe even V) flux and do NOT fit for the
source(s) Q and U. For a single calibration source,
image models may be used and/or PMODEL if PMODEL(1) >
0. One of these must be done for single-source files.
For multi-source files, the source table may be used
to provide the full model instead and is the only
place to get models for more than one calibration
source.
= 2 => use the I, Q, U, V model contained in an attached
CP table (version given by CPARM(4)).
PMODEL.....See first DOMODEL above. A single component model to be
used instead of a CLEAN components model; if PMODEL(1) >
0 amd DOMODEL > 0, then use of this model is requested
when a single source is being used
PMODEL(1) = I flux density (Jy)
PMODEL(2) = Q flux density (Jy)
PMODEL(3) = U flux density (Jy)
PMODEL(4) = V flux density (Jy)
PMODEL(5) = X offset in sky (arcsec)
PMODEL(6) = Y offset in sky (arcsec)
PMODEL(1) must be > 0 to give an I flux for single-source
data sets independent of the value of DOMODEL. The rest
of PMODEL must be set intentionally when DOMODEL > 0 and
IN2NAME is blank. PMODEL is ignored if IN2NAME or
IN2CLASS is not blank.
SPECPARM...The spectral index in I, Q, U, and V for each source to
be used mostly when DOMODEL > 0. In the continuum case,
it is applied only when PMODEL is used. In the line
case, it is also applied in I, Q, U, and V when PMODEL is
used. When DOMODEL <= 0, the I flux is still needed to
solve for Q and U, so the I portion is still used. Thus
meaningful values for SPECPARM(1,i) are helpful. Note
that i is a sequential number assigned to all CALSOUR in
the order of the source numbers in the source table (not
their order in the adverb CALSOUR).
DOSCALE....= -1 => do not use spectral index in the task
>= 0 => use spectral index from SPECPARM or solve for
spectral indices using those for known sources
or fluxes in the SU table
= 2 => when using SU table fluxes, solve for a curvature
as well as spectral index.
SOLINT.....Time interval to average data before determining
correction in minutes. 0 => 5 min. If there is a
single calibrator scan, the task will reset SOLINT to 0.333
times the length of that scan. SOLINT is an interesting
parameter to explore if having difficulties with solutions.
SOLTYPE....Solution type:
'APPR' => linear approximation,
'RAPR' => linear approximation but allowing
resolved sources.
'ORI-' => orientation- ellipticity allowing
resolved sources
other values = 'APPR'
Note: 'RAPR' and 'ORI-' require nonstandard
calibration procedures, see EXPLAIN PCAL for
details.
SPECTRAL...> 0 => do the solution as a function of spectral channel
within each IF. Answers go into a PD table and
source models, if they are part of the solution,
go into the CP table.
<= 0 => do the solution only as a function of IF.
Antenna polarizations are placed into ther
antenna table and source models (if DOMODEL<=0
and CPARM(2)>0) go in the SU table.
INTPARM....After the data are calibrated with all parameters
including SMOOTH, they may be smoothed in frequency
further using the INTPARM selected function while taking
into account ICHANSEL. This is to provide better S/N
than one would have with unsmoothed single channels.
INTPARM(1) = type of smoothing to apply:
0 => no smoothing
1 => Hanning
2 => Gaussian
3 => Boxcar
4 => Sinc (i.e. sin(x)/x)
INTPARM(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 INTPARM(1) =
1 - 4.
INTPARM(3) = the diameter over which the convolving
function has value - in channels. Defaults: 1, 3, 1, 4
times INTPARM(2) used when input INTPARM(3) <
INTPARM(2) (after application of defaults).
To avoid spectral shifts, all functions are applied
symmetrically. Boxcar of width "2" actually does 3
channels averaging, for example, channels 9, 10, and 11
for output channel 10. Width 3 does the same. Width 4
is needed to include channels 8 and 12. This is different
than boxcar with SMOOTH.
PRTLEV.....If this value is larger than 0.0 then the
intermediate diagnostics and solutions will be
given on the monitor terminal and the message file.
REFANT.....Reference antenna to use. If none is specified, the task
will pick the lowest numbered one with the most samples.
Previous methods that allowed "none" gave suspect
results and could not measure uncertainties.
BPARM......Task enrichment parameters (SOLTYPE='ORI-' only):
(1) If BPARM(1) is greater than 0 then the initial
values for the feed ellipticity and orientation
will be that for perfect RCP and LCP feeds.
Otherwise the values from the AN table, if any,
will be used.
(2) > 0 => suppress error calculation - which can be
meaningless and can even get into an infinite loop.
(3) If BPARM(3) is greater than 0 then fit for the
R-L phase difference using BPARM(4) as the
initial value. This option is only useful if at
least one source has a fixed polarization
model.
(5) If BPARM(5) is greater than 0 then solve for
Vpol.
(6) If BPARM(6) is greater than 0 then fix the value of
the orientation of the first polarization of the
reference antenna to 0. (REFANT > 0)
(7) If BPARM(7) is greater than 0 then fix the value of
the orientation of the second polarization of the
reference antenna to 0. (REFANT > 0)
(8) If BPARM(8) is greater than 0 then fix the
orientations at the values in the AN table.
(9) If BPARM(9) is greater than 0 then fix the
ellipticities at the values in the AN table.
(10) if (BPARM(10) is greater than 0 then fix the
source polarization parameters to the values
given in the SU table.
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. This is usually NOT the case for
VLA data.
(2) > 0 => update the source table with the fit source
parameters. Note, because of the RL phase
difference, the Q and U fit may not be correct.
(3) In SPECTRAL > 0 mode, channels flagged either by
the usual flags or by ICHANSEL and not recovered by
smoothing with INTPARM will end up with solutions
which are 0.0. The task will replace these incorrect
ones with interpolated or extrapolated ones unless
CPARM(3) > 0.
(4) If DOMODEL = 2, the CP table version number to be
used. 0 => the highest.
(5) The linear polarization model is not believed to work
correctly. If you want to try it on your linearly
polarized data, set CPARM(5) > 0. If DOMODEL is true
and the source polarization is zero, the circular
model (APPR) is thought to work well.
(6) Not currently used.
(7) An initial guess may be found by reading the AN table
(SPECTRAL <= 0) or the PD table if CPARM(7) > 0. If
the latest values in the AN table or the highest
version of PD table are suspect, this is not a good
idea. But it may help convergence if those values
are reasonable.
(8) If CPARM(8) is greater than 0 then it will be
used as the maximum number of iterations for
the SOLTYPE='ORI-' solution.
(9) If CPARM(9) is greater than 0 then it will be
used as a convergence criterion for the
SOLTYPE='ORI-' solution. A value of 0 causes a
default criterion to be used.
(10) If CPARM(10) is greater than 0 then it will be
used as a convergence criterion for the
SOLTYPE='ORI-' solution. A value of 0 causes a
default criterion to be used.
DPARM (1) Missing Faraday rotation values can cause blanked
values to appear later on. If DPARM(1)>0, the data
will not be flagged for this, i.e. PCAL will assume
that the correction is unimportant.
BADDISK....Disk numbers on which scratch files are not to
be placed.
EXPLAIN SECTION
PCAL: Task to determine effective feed polarization parameters.
Documentor: W. D. Cotton (preliminary version).
Related Programs: CALIB, LISTR, SPLIT, CLCOR, LPCAL, SPCAL
Use of PCAL is also described in EXPLAIN CALIBRAT.
Polarization calibration of synthesis array visibility data
consists of two distinct parts: 1) the determination of the effective
response of the feed 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. PCAL determines the effective response of the feeds, the
polarization of unknown calibrator(s) and stores this information in
the AN and SU tables. Routines which can apply calibration tables can
then be instructed to apply the polarization corrections by setting
the adverb DOPOL=1.
If the systematic phase delay differences between the right and
left hand systems is time variable (e.g. variable ionispheric Faraday
rotation) then these effects need to be removed before running PCAL
(see CLCOR). If the systematic phase offsets are essentially constant
then they may be removed after running PCAL by using RLDIF or LISTR
with STOKES='POLC'; DOPOL=1 and appropriate calibration on a source
with known polarization angle to determine the phase offsets and then
applying them using CLCOR and OPCODE='POLR'. Note: this later method
will modify the AN table as well as the CL table.
SINGLE SOURCE FILES:
In principle, PCAL can work on a single-source file followed by
SPLIT to apply the correction. However, in practice, it is preferable
to convert all of the files involved to multi-source files using task
MULTI followed by DBCON with DOARRAY=TRUE to glue the files together.
DBCON should be followed by INDXR to create an index table. The flux
densities of the calibrator source(s) to be used by PCAL should be
entered in the Source (SU) table using SETJY. Also, see the section
on parallactic angles.
After PCAL has been successfully run and any phase offsets have
been removed, then single source, polarization corrected files can be
obtained using SPLIT with DOPOL=1; DOCAL=TRUE and
GAINUSE=(relevant CL table).
PARALLACTIC ANGLE:
When used with SOLTYPE='APPR', PCAL expects that the input data
has NOT had the parallactic angle removed from the phase of the cross
hand visibility data. The AIPS software makes this correction when
the instrumental corrections are applied, i.e., when DOPOL=1. If
this correction has been made (e.g. any data which has been through
the VLA Dec-10) then PCAL will give invalid results. (VLA data read
by the AIPS program FILLM has not had this correction made and can be
fed to PCAL as is.)
If SOLTYPE='RAPR' or 'ORI-' then the parallactic angle correction
SHOULD have been made before phase calibration. Also for
SOLTYPE='ORI-' the phase reference antenna used in the calibration
process should be the same as REFANT given to PCAL.
If you are unsure about this correction then run LISTR with
OPTYPE='MATX'; STOKES='POLC'; DPARM(1)=1; SOURCES='(your calibrator
source)'; DOPOL=-1; DOCAL=TRUE; GAINUSE=appropriate CL (or SN for
single source files) table. The parallactic angle may be determined
from LISTR using OPTYPE='GAIN'; DPARM(1)=9. If the parallactic angle
correction has NOT been applied then the phase of the RL correlations
will vary approximately as the negative of the parallactic angle. If
the range of phase of the RL correlator is much less than the range of
the parallactic angle then the parallactic angle correction has been
made and needs to be removed.
REMOVAL OF THE PARALLACTIC ANGLE CORRECTION:
Before removing the parallactic angle correction by modifying the
CL table it is best to copy the current working CL table to the next
available sequence number with TACOP and modify the copy. If
something goes wrong you can start over using the previous version
(also CLCOR refuses to modify CL table version 1). Remember to set
the OUTNAME etc. in TACOP.
If you have determined that the parallactic angle correction has
already been made to your data then it can be removed using CLCOR with
SOURCES=''; STOKES=''; BIF=1; EIF=0 TIMERANG=0; ANTENNAS=0; CLCORP=0;
SUBARRAY=subarray number; GAINVER=CL table no. to modify;
OPCODE='PANG'; BPARM=0. CLCOR will remove the parallactic angle
correction by rotating the phases of the complex gains to be applied
the the data. If there is more than one subarray present then CLCOR
must be run for each subarray; the number of subarrays can be
determined from the number of AN tables present in the file as shown
by IMHEAD. After this correction is made, then PCAL should be run with
DOCAL=TRUE and GAINUSE=(the CL table modified by CLCOR). To add the
parallactic angle correction for SOLTYPEs RAPR and ORI-, use PANG in
CLCOR with CLCORP(1)=1.
CALIBRATION DATA MEASURED WITH LINEARLY POLARIZED FEEDS
Data obtained with systems using linearly polarized feeds
(especially the Australia Telescope) may determine the instrumental
polarization using a linear approximation. This type of solution is
indicated in the AN table as POLTYPE='X-Y LIN '. The standard
calibration routines will apply these corrections if adverb
DOPOL=1.
NOTE: X-Y linear data (STOKES = -5) MUST be polarization before
attempting to use Q, U or V polarization.
SOLTYPE:
SOLTYPE='APPR' does a fits a linear approximation to the feed
parameters and source polarizations using one or more unresolved
calibrator sources.
SOLTYPE='RAPR' also fits a linear approximation to the feed
parameters but allows the sources to be resolved. The method
explicitly assumes that the polarization structure has constant
position angle and is the same as the total intensity structure
with a scaling factor. This assumption will seriously break
down if the source is significantly resolved (larger that 1 - 2
synthesized beam widths).
SOLTYPE='ORI' fits for feed ellipticity and orientation.
Since this procedure can be quite expensive in compute time it
should only be used in cases where the linear approximation
breaks down (more than a few percent instrumental polarization).
This method can optionally solve for the R-L phase offset if a
polarization model is given.
USING RESOLVED CALIBRATORS
Although SOLTYPEs 'RAPR' and 'ORI-' allow resolved
calibrators the assumptions inherent in their use break down in
cases of significant resolution. In this case it may be useful
to make a few iterations of PCAL, image, and deconvolve to
develop a accurate model to give to PCAL (IN2NAME etc.).
For calibrators that are very resolved, and/or have polarization
structure, PCAL will not work well. LPCAL was designed for
this case.
LPCAL and SPCAL
LPCAL allows you to put in multiple models for multiple source
components. These are allowed to have different polarizations.
If you have VLBI data it is likely you will have a resolved
calibrator with varying polarization structure. If you have
spectral line data or if you suspect that the D-terms vary
across the frequency band try using SPCAL. However SPCAL does
not allow the polarization to vary across the calibrator (like
LPCAL). Both LPCAL and SPCAL will work for unpolarized
calibrators.