AIPS HELP file for IMSCAL in 31DEC24
As of Wed Apr 24 2:13:13 2024
IMSCAL: Determines antenna complex gain from large image
INPUTS
Input uv data.
INNAME UV file name (name)
INCLASS UV file name (class)
INSEQ 0.0 9999.0 UV file name (seq. #)
INDISK 0.0 9.0 UV file disk drive #
input image
IN2NAME Large image name (name)
IN2CLASS Large image name (class)
IN2SEQ Large image name (seq #)
IN2DISK Large image name (disk)
BLC 0.0 4096.0 Bottom left corner of image
0=>1
TRC 0.0 4096.0 Top right corner of image
0=>max allowed
ICUT 0.0 Include all points > ICUT in
absolute value only
FLUX Discard all points < FLUX
NX 1.0 Number panels in X
NY 1.0 Number panels in Y
OUTNAME name to use for temporary
files (CC, OOSUB output)
OUTDISK Disk to put subimages/CCs
Solution control adverbs:
DOKEEP -1.0 1.0 > 0 -> keep divided us data
Data selection (multisource):
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 = center 75 percent of band.
ANTENNAS Antennas to select. 0=all
DOFIT Subset of ANTENNAS list for
which solns are desired.
ANTUSE Mean gain is calculated
(CPARM(2)>0) using only the
listed antennas. See explain.
UVRANGE Range of uv distance for full
weight
WTUV Weight outside UVRANGE 0=0.
WEIGHTIT 0.0 3.0 Modify data weights function
Cal. info for input:
CMETHOD Modeling method:
'DFT','GRID',' '
REFANT Reference antenna
SOLINT Solution interval (min)
SOLSUB Solution subinterval
SOLMIN Min solution interval
APARM General parameters
1=min. no. antennas
3 > 0 => avg. RR,LL
5 > 0 => avg. IFs.
6=print level, 1=good,
2 closure, 3 SNR
7=SNR cutoff (0=>5)
8=max. ant. # (no AN)
9 > 0 => pass failed soln
10 < 99 cal output weights
Phase-amplitude Parameters:
DOFLAG Flag on closure error?
SOLTYPE Soln type,' ','L1','GCON',
'R', 'L1R', 'GCOR'
SOLMODE Soln. mode: 'A&P','P','P!A',
'GCON',
SOLCON Gain constraint factor.
MINAMPER 0.0 Amplitude closure error
regarded as excessive in percent
MINPHSER 0.0 Phase closure error regarded
as excessive in degrees
NORMALIZ -1.0 4.0 >0 => normalize gain:
1 globally, 2 by subarray,
3 by subarray,IF
4 by subarray,IF,pol
CPARM Phase-amp. parameters
1 = Min el for gain
normalization (deg)
2 > 0 normalize w median
else use mean
3 avg. amp. closure err
4 avg. ph. closure err
5 = 1 vector average
channels, scalar avg
between times
>= 2 scalar average
6 limit clipping in robust
7 limit display of closure
errors
ANTWT Ant. weights (0=>1.0)
GAINERR Std. Dev. of antenna gains.
BPARM Task enrichment parameters
(1) Antenna diameter (m)
0 -> no correction
(2) Omit CC options
(3) spectral index radius
0 -> no correction
FQTOL Frequency tolerance in kHz
(primary beam & spec index)
IN3NAME Spectral index image name
IN3CLASS Spectral index image class
IN3SEQ Spectral index image sequence
number
IN3DISK Spectral index image disk
IN4NAME Spectral curvature name
IN4CLASS Spectral curvature class
IN4SEQ Spectral curvature sequence
number
IN4DISK Spectral curvature disk
BADDISK Disk no. not to use for
scratch files.
HELP SECTION
IMSCAL
Task: IMSCAL is a procedure that combines IM2CC, OOSUB, CALIB, and
finally TACOP. IM2CC breaks a large (usually CASA) image into
pieces, making both an image and a Clean components table for
each piece. This is needed to allow AIPS to use its geometries
while CASA has used the W-projection geometry. OOSUB divides
the data for a calibration source by the model from these
pieces plus options to permit frequency-dependent primary beam
and spectral index corrections. CALIB then determines an
IF-dependent, spectral-channel independent antenna gain and
phase, using the divided data. A solution (SN) table is
written and copied to the input uv file by TACOP. Finally the
temporary, divided data set is deleted (optionally) and the
image pieces are deleted..
This procedure does not apply data selection and calibration
adverbs to the input data set. You must apply these with SPLIT
or SPLAT (or other tasks) to make a data set consisting solely
of the edited/calibrated data that you wish to self-cal.
IMSCAL makes a number of temporary files all of which are
assigned specific names including sequence number 1. It will
check for the presence of any of these on your disks before
doing work. If any are present, IMSCAL will list them and
quit, allowing you to rename them or delete them.
This procedure is obtained by entering RUN OOCAL.
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.
**** Model image in Jy/pixel (not convolved with beam) ****
IN2NAME....Input image file name (name). Standard defaults.
IN2CLASS...Input image file name (class). Standard defaults.
IN2SEQ.....Input image file name (seq. #). 0 -> highest.
IN2DISK....Disk drive # of input image. 0 -> any.
BLC........The bottom left-hand pixel of the input image which
becomes the bottom left corner of the subimage from
which the NX x NY panels are taken. 0 -> 1.
TRC........The top right-hand pixel of the input image which
becomes the top right corner of the subimage from which
the panels are taken. 0 -> max allowed value.
ICUT.......CC components are made only from pixel values greater in
absolute value than ICUT and
FLUX.......CC components are made only from pixel values greater
than FLUX (in actual value). Thus FLUX=0 cuts off all
negatives.
NX.........The X axis is divided into NX nearly equal panels.
NY.........The Y axis is divided in NY nearly equal panels.
Be sure to make enough to account for any W term issues.
OUTNAME....The output subimages are stored on OUTDISK with this name
parameter. The OUTSEQ=1 and OUTCLASS=IMCnnn. The output
from the OOSUB step in OOCAL also uses this name with
OUTCLASS 'OOCAL1'.
OUTDISK....The output subimages and CC files are put on OUTDISK, the
OOSUB output file is put on INDISK. It is better to
avoid Lustre disks for the CC files.
DOKEEP.....> 0 => keep the file produced by OOSUB containing the
input data divided by the model and the SN table
produced by CALIB
<=0 => delete this temporary file after TACOP.
The following may be used for all data files (except as noted):
ICHANSEL...Array of start and stop channels plus a channel increment
and IF, used to select the channels to be averaged. 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. Up to 20 groups of start, stop
and increment channel numbers plus IF numbers can be
specified. The default (ICHANSEL = 0) is to average the
center 75 percent of the band, i.e.
ICHANSEL(1,1) = (# channels)/8 + 1
For example: # channels=16 => ICHANSEL(1,1)=3
ICHANSEL(2,1) = (# channels + 1)*7/8
For example: # channels=16 => ICHANSEL(2,1)=14
ICHANSEL(3,1) = 1
ICHANSEL(4,1) = 0 (meaning all IFs).
If ICHANSEL describes averaging explicitly for some IFs,
but skips other IFs, then the center 75 percent of the band is
averaged for the skipped IFs. For example:
ICHANSEL=2,6,1,2 => The channels 2-6 will be averaged for
IF=2 and the center 75 percent of the band will be averaged for
the rest of the IFs.
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.
DOFIT......A list of the antennas for which solutions should be
determined. Only those antennas listed in DOFIT will be
solved for; all data selected via ANTENNAS will be used to
form the solutions. If any antenna number in DOFIT is
<= -1, then DOFIT is taken as the list of antennas for
which no solution is desired; a solution is found for all
antennas not in DOFIT. Note that the REFANT, if specified,
will _not_ be solved for even if it appears in the DOFIT
list. Selection via DOFIT can be disabled by setting DOFIT
= 0 which defaults to solving for all antennas.
NOTE: THIS OPTION MUST NOT BE USED UNLESS YOU UNDERSTAND
IT FULLY. Basically, it should be used to solve for the
gains of "poor" antennas after the "good" antennas have
been fully calibrated. Antennas included in ANTENNAS but
not in DOFIT are assumed to have a complex gain of (1,0)
and the gains produced will be very wrong if this is not
the case. See HELP DOFIT.
ANTUSE.....A list of the antennas to be used in the calculation of
the mean gain modulus (NORMALIZ>0). If any number is
negative then all antennas listed are NOT to be used
to determine the gain normalization and all others are.
All 0 => use all. It can be useful to limit the
antennas used for the gain normalization to those with
good a priori calibration, especially when using
VLBI-style calibration based on system temperatures
and gains. This prevents the flux scale from being
dragged up or down by poorly calibrated antennas
including antennas subjected to bad weather. The
normalization factor determined using the ANTUSE
antennas is applied to all antennas.
SUBARRAY...Subarray number to use. 0=>all.
UVRANGE....The range of uv distance from the origin in
kilowavelengths over which the data will have
full weight; outside of this annulus in the uv
plane the data will be down weighted by a factor
of WTUV.
WTUV.......The weighting factor for data outside of the uv
range defined by UVRANGE.
WEIGHTIT...If > 0, change the data weights by a function of the
weights just before doing the solution. Choices are:
0 - no change weighting by 1/sigma**2
1 - sqrt (wt) weighting by 1/sigma may be more stable
2 - (wt)**0.25
3 - change all weights to 1.0
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
difference uv sampling it is best to use 'DFT'
The following control how the solutions are done, if you don't
understand what a parameter means leave it 0 and you will
probably get what you want.
REFANT.....The desired reference antenna for phases.
SOLINT.....The solution interval (min.)
0 => scan average for multi-source,
0 => 10 s for single source amp-phase solns. (VLA)
0 => 10 min for delay-rate solutions (VLBA).
OOCAL tries hard to make equal integrations within each
scan but that is a problem that lacks a general solution.
You can help by careful choice of SOLINT: assume you have
data every 10 seconds. Then, to get 1 sample per
solution, set SOLINT=9/60. To get 2 per solution, set
SOLINT=19/60, 3 per solution SOLINT=29/60. Each averaged
interval will start with an actual data sample and will
end just before the first sample at a time greater than
the start + SOLINT + 0.1s. At the end of the scan, the
end time can be increased by up to 0.6 * SOLINT to
prevent short final integrations. For calibration that
is not self-calibration, note that the 2-point
interpolation will use ONLY the last integration of a
calibrator scan with the first integration of the next
calibrator sacn. That is why the initial calibration
normally uses scan averages for the calibrator sources.
-------------------------
If the times in your data set are not at regular
intervals due to flagging and averaging, you must be
careful with SOLINT. To get all data in 10 seconds (from
0 through 9.999) set SOLINT to 9.999/60. Use of 1 sec
will do odd things with the records at odd times.
-------------------------
SOLSUB.....The begin time for the next interval in advanced from the
current one by SOLINT / SOLSUB where 1 <= SOLSUB <= 10.
0 -> 1. This is to produce solutions at sub-intervals of
SOLINT based on SOLINT length of averaging.
SOLMIN.....Minimum number of subintervals to be used in a solution.
0 -> SOLSUB.
APARM......General control parameters.
APARM(1)...Minimum number of antennas allowed for a solution.
0 => max (3, min (6, Numant/2))
APARM(2)...If > 0 then the input data has already been
divided by a model; only solutions will be
determined.
APARM(3)...If > 0 then average RR, LL
APARM(5)...If > 0 then make a combined solution for the IFs;
if <= 0 then make separate solutions.
APARM(6)...Print flag, 0=none, 1=time, closure error statistics,
2=individual closure failures (exceeding both CPARM(7)
sigma and either MINAMPER or MINPHSER), 3=some additional
info including the antenna signal to noise ratio,
4=solutions, 5=data too.
APARM(7)...The minimum allowed signal-to-noise ratio. 0 => 5
APARM(8)...If there is no antenna (AN) table with the input file
then the maximum antenna number in the file should be
entered in APARM(8).
APARM(9)...When solutions fail or there is insufficient data and
APARM(9) > 0 then (1,0) is written to the SN table. This
will preserve the previous calibration but this option
should be used with extreme care.
APARM(10)..When writing a single-source output file, calibrate the
output weights except when either DOCAL > 99 or
APARM(10) > 99 or both.
Phase-amplitude parameters:
DOFLAG.....If DOFLAG > 0, those baselines with excessive closure
error will be added to a new flag table along with all
flags previously in FLAGVER. If DOFLAG <= 0, no new data
flags are generated and no flag table is written. In
either case, all SOLTYPEs below check the data for
closure error and will report the fractions for which the
closure error exceeds abs(DOFLAG) times the rms closure
error. Defaults for the reporting level are:
0.0 < DOFLAG < 2.0 -> 2.5
-1.0 < DOFLAG <= 0.0 -> -2.5 if APARM(6) = 0
-99.0 if APARM(6) >= 1
Note that this checking of closure error is in addition
to the checking done under control of MINAMPER, MINPHSER,
and APARM(6) which do not flag the data. Note that data
marked as "bad" by the DOFLAG test will not be checked
with the APARM(6) controlled tests. The default on
DOFLAG=0 is then meant to enable better printing of
closure errors when not doing any actual flagging.
SOLTYPE....Solution type:
' ' => normal least squares,
'R ' => as ' ' with robust iteration
'L1 ' => L1 solution; a weighted sum of the moduli
of the residuals is minimized.
The computed gain solutions are less
influenced by wild data points, but there
is some loss of statistical efficiency.
See [F.R. Schwab, VLA scientific Memo #136]
for further details.
'L1R ' => as 'L1' with robust iteration
'GCON' => least squares which may include gain
constraint.
'GCOR' => as 'GCON' with robust iteration
SOLTYPE (other than the R) is ignored when the DOFIT
option is used. The robust versions iterate the
solution, discarding data that does not fit the current
solution well enough. They should be less disturbed by
bad data, but will be slower.
SOLMODE....Solution mode:
'A&P ' => amplitude and phase.
'P ' => phase only
'P!A ' => phase only (no amplitude information)
'GCON' => amplitude and phase with constraints on
amplitude. This mode requires setting
SOLTYPE='GCON', uses GAINERR and
SOLCON may be used.
' ' => 'A&P ' for multisource (raw) data,
=> 'P ' for single source data.
SOLCON.....Gain constraint factor; a value larger than 0 will
increase the strength of the amplitude constraint
in gain constrained solution with SOLMODE='GCON'
MINAMPER...Amplitude closure error regarded as excessive in per cent.
If APARM(6) > 0, summaries of the number of excessive
errors by antenna are printed and, if APARM(6) > 1, up to
1000 of the individual failures are printed. 0 => do not
check or report "excessive" closure errors of any sort.
Note that amplitude closure errors are accumulated using
logarithms so that gains of 0.5 and 2.0 are both errors of
100 percent.
MINPHSER...Phase closure error regarded as excessive in degrees.
If APARM(6) > 0, summaries of the number of excessive
errors by antenna are printed and, if APARM(6) > 1, up to
1000 of the individual failures are printed. 0 => do not
check or report "excessive" closure errors of any sort.
NORMALIZ...If > 0, constrain the mean gain modulus of the calibration
applied to be unity. This is mostly used in self
calibration. If NORMALIZ = 1, the mean is over all IFs,
antennas, polarizations and subarrays. If NORMALIZ =2,
it is the same except separated by subarray. If NORMALIZ
= 3, it is averaged over all antennas and polarizations
but separated by IF and subarray. NORMALIZ=4 also
separates by polarization. If you select NORMALIZ= 1,
the global scaling factor is written to the header of the
SN table. For options 2 - 4, the SN table is re-written
with the final scaling applied. If = 0, substitute
CPARM(2) which was previously used for this information.
CPARM......Phase-amplitude parameters.
CPARM(1)...Minimum elevation in degrees for the solutions used to
constrain the mean gain modulus. 0 or >80 => no
constraint (actually -100 is used).
CPARM(2)...If NORMALIZ > 0, CPARM(2) controls whether a mean value
(CPARM(2) <= 0) or a median value (CPARM(2) > 0) is used.
CPARM(3)...If > 0, the values of any amplitude closure errors whose
average absolute percentage value exceeds CPARM(3) will be
printed if APARM(6) > 0.
CPARM(4)...If > 0, the values of any phase closure errors whose
average absolute value exceeds CPARM(4) degrees will be
printed if APARM(6) > 0..
CPARM(5)...If > 0 then the amplitudes will be scalar averaged when
averaging across times before determining the solutions.
The averaging of spectral channels will be a vector
average unless CPARM(5) > 1.5. Vector averaging is
preferred to avoid the Ricean bias in amplitudes, but not
when phase instability will make the signal incoherent.If
the atmospheric phase is very unstable, then it should be
fine at any one time but may require the scalar averaging
between times.
CPARM(6)...The robust solution method discards data more than
f(iter) * rms(iter)
from the current solution to find the iter+1 solution
f(iter) = max (g(iter), CPARM(6)) and
g = 7.0, 5.0, 4.0, 3.5, 3.0, 2.8, 2.6, 2.4, 2.2, 2.5.
Thus CPARM(6) can be used to limit the discarding to less
restrictive values.
CPARM(7)...The printing of individual closure errors occurs only if
APARM(6) >= 2 and the errors exceed MINAMPER percent in
amplitude and/or MINPHSER degrees in phase. That
printing is also limited to those errors that are more
than CPARM(7) times the 1 sigma expected error (based on
the data weights). 0 -> 2.5. If you want no limit,
set CPARM(7) to something like 0.001.
ANTWT......Antenna weights. These are additional weights
to be applied to the data before doing the
solutions, one per antenna. Use PRTAN to
determine which antenna corresponds to each
antenna number.
0 => 1.0
GAINERR....Estimates of the standard deviation of the modulus of the
gains for each antenna. These are used ONLY if SOLMODE
and SOLTYPE='GCON'. The solution will attempt to make
the standard deviation of the modulus of the antenna
gains match these values so accurate values are
essential.
BPARM......Correction control parameters (SEE EXPLAIN IMAGR):
(1) If > 0 then make frequency dependent primary beam
corrections assuming an antenna diameter of IMAGRPRM(1)
meters. Note that VLA and ATCA arrays (TELESCOPE
header parameter) use the default primary beam
parameters defined elsewhere in AIPS, while other
antennas actually use IMAGRPRM(1) as the diameter of a
"standard" telescope. See FQTOL below also.
(2) If BPARM(1) > 0, you may omit selected CCs from the
operation based on position:
BPARM(2) <= 0 : Include all CCs
= 1 : Omit CCs within the main beam at
all frequencies
= 2 : Omit CCs within the main beam at
some frequncies
= 3 : Omit Ccs outside the main beam at
some frequencies
= 4 : Omit CCs outside the main beam at
all frequencies
(3) 1 => use a spectral-index image represented in
IN3NAME, IN3CLASS, IN3SEQ, IN3DISK below to correct the
Clean component model for each channel. IN4NAME et al
will also be used as a curvature image iff IN3NAME are
specified.
BPARM(3)-0.5 is used as a radius in pixels over which
the spectral index image is averaged. When it is small
(0 < BPARM(3) <~ 1), the spectral index is interpolated
rather than averaged. See FQTOL below as well. When
doing spectral index, the primary beam correction
(BPARM(1)) costs very little extra. This parameter is
IMAGRPRM(17) in IMAGR.
FQTOL......Frequency tolerance in kHz. Spectral channels with FQTOL
are handled together (use the same average CC model) when
applying the primary beam and spectral index
corrections. Default is to do each channel separately
which can take a long time.
IN3NAME....Image name of spectral index image; no default.
IN3CLASS...Image class of spectral index image; no default.
IN3SEQ.....Image sequence of spectral index image; 0 -> highest.
IN3DISK....Disk of spectral image image; 0 -> any.
IN4NAME....Image name of spectral index curvature image; no default.
Curvature images should be base 10 rather than base e -
they differ by a factor of 2.3. Also the reference
frequency is 1.0 GHz. These are changes done 2010-07-13.
IN4CLASS...Image class of spectral index curvature image; no
default.
IN4SEQ.....Image sequence of spectral index curvature image;
0 -> highest.
IN4DISK....Disk of spectral curvature image image; 0 -> any.
BADDISK....Disk numbers on which scratch files are not to
be placed.
EXPLAIN SECTION
OOCAL: Task to determine antenna gains from calibrator data
Documentor: A.H.Bridle
Related Programs: CLCAL, LISTR, SPLIT, UVFLG
This task is the central AIPS routine for calibrating multi-
source uv data sets using observations of calibration sources
that can either be assumed to be point sources or have well
determined structures.
OOCAL determines antenna voltage gain solutions (amplitude
and/or phase) from data for calibrator sources with well known
flux densities, positions and structures. It is the equivalent
of the VLA ANTSOL (with additional options). Solutions
determined by OOCAL under control of the APARM, CPARM and DPARM
parameters are written to the solution (SN) extension table of
the input uv data set. Solution tables may be merged, smoothed
and interpolated into calibration (CL) tables for multi-source
uv data files using CLCAL.
OOCAL may also be run on files containing data for only one
source, for self calibration.
To run OOCAL, you should specify at least:
The input uv data file (INNAME, INCLASS, INSEQ, INDISK).
The CALSOURces to be used for determining antenna gains, or
leave CALSOUR blank and specify a CALCODE and/or qualifier.
REFANT, the reference antenna for the solution (choose an
antenna with good signal to noise that was present through
as much of the observing as possible).
The defaults are set so that running OOCAL on a multi-
source uv data file setting only these inputs will make
a solution file for all IFs in the data over the entire
time range using the highest-numbered flag file. All
antennas will be calibrated for amplitude and phase, using
data from the entire uv range. All antennas will be equally
weighted. Point source models will be assumed for the
calibration sources. The solution will be written to an
SN table.
Other useful options
====================
Use APARM(6)=3 to list the signal to noise ratio at each
antenna solution. Solutions with signal to noise below
5:1 are probably not meaningful and will be discarded by
the default setting of APARM(7). You may wish to apply
more stringent criteria with APARM(7).
Use SOLTYPE and SOLMODE='' to solve for both amplitude and
phase solutions simultaneously with no constraints on amplitude,
SOLTYPE and SOLMODE='GCON' for amplitude and phase with
constraints set by GAINERR and SOLCON. Amplitude solutions
for point source models in multisource files will be based on
the flux densities entered for the sources in the source (SU)
table extension of the data set using task SETJY.
Use UVRANGE and WTUV to weight different uv ranges
differently (or to restrict the solution to some uv range
-- WTUV = 0 is read as zero weight).
"CLEAN" COMPONENT MODELS
========================
OOCAL does not restrict you to the use of point source models
for your calibrators. Use IN2DISK, IN2NAME, IN2SEQ, INVERS,
NCOMP and NMAPS to specify a CLEAN component model for the
field or fields around a calibrator and specify that
calibrator in SOURCE. OOCAL will create an SN table for that
calibrator alone. This SN table may then be merged with SN
tables for other calibrators produced by other runs of OOCAL,
when the SN tables are smoothed and interpolated into a CL file
by CLCAL.
FLUX CALIBRATOR MODELS
======================
This is actually a subsection of CLEAN COMPONENT MODELS above.
You are strongly encouraged to use the flux calibrator models
available for all the primary flux calibrators (3C138, 3C147,
3C286 and 3C48). Using calibrator models removes the need set
UVRANGE and ANTENNAS. To see what calibrator models are available
in AIPS type CALDIR, to read them in use the task CALRD. CALRD
loads in the selected model as an image file. Then specify this
image in IN2DISK, IN2NAME, IN2CLASS and IN2SEQ. OOCAL will
recognize these images as standard calibrator models and scale
the clean components in the central part of the field with the
flux in the SU table.
RESETTING YOUR CALIBRATION
==========================
The first version of the CL table attached to your uv data set
is protected from modification in CLCAL, so
that you can easily "undo" all calibration steps that have
taken place within AIPS. To reset your calibration, delete all
CL tables with version numbers >1, and delete all SN tables.
POLARIZATION considerations (thanks to Robert Braun)
====================================================
The VLA measures approximately (neglecting the leakage terms):
RR=I+V, LL=I-V, RL=Q+iU, LR=Q-iU
The WSRT uses linear polarization but the equatorial mounts mean that
the feed orientation remains constant relative to the sky (no
parallactic angle change at all). Thus, the WSRT measures
approximately:
VV=I-Q, HH=I+Q, VH=-U+iV and HV=-U-iV
If you compare these two sets of equations, you see that they have a
lot in common. If you simply pretend that you have measured
(RR,LL,RL,LR) by changing the Stokes value from -5 to -1 in the header
with PUTHD, you're almost in business, except you have -Q in the place
of V, -U in the place of Q, and V in the place of U. This is fine for
most things, since you just have to request a slightly different
parameter from the one you really mean.
The biggest hassle comes from amplitude calibration of linearly
polarized sources, like:
3C286 (near 1.4 GHz) which has (I,Q,U,V)=(14.65,0.56,1.26,0.00) Jy
3C138 (near 1.4 GHz) which has (I,Q,U,V)=(8.30,0.63,-0.17,0.00) Jy
Since Q is non-zero, it means that the VV and HH correlations do not
correspond to the same real flux density (ie. VV=I-Q and HH=I+Q).
Now, since RR=I+V and LL=I-V, the idea is to have OOCAL use the
Stoke's V from the SU table. The, for WSRT data, one can fudge the
right behavior, by putting the source's actual value of -Q in place
of V in the SU table.
This shouldn't harm any VLA users, since Stoke's V is near enough zero
for most sources anyway. And if it were non-negligible it should be
taken along in any case, since it really does affect the RR and LL
correlations, and therefore the derived gain of the R and the L IFs.