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First make IQUV images of the polarization calibrator 3C 286 to
determine the polarization model (a more detailed description of
imaging is given in the next section). First, start the imager tool
and click on the Function Group button to the left of the GUI.
Choose the option ALL function groups so you can see all
possible functions available within the imager tool. Then
select the 3C 286 data, define the image characteristics and weighting
scheme. Finally, make a simple, un-CLEANED image called 3c286.cal1.im
and write it to disk:
imgrC:=imager(filename='ap366.ms'); # Start the imager tool
imgrC.setdata(mode='none', # Select continuum data for
fieldid=11); # for 3C 286.
imgrC.setimage(nx=512, # Set up imaging parameters.
ny=512,
cellx='0.1arcsec',
celly='0.1arcsec',
stokes='IQUV',
fieldid=11);
imgrC.weight(type='uniform'); # Choose weighting
# e.g. uniform, natural.
imgrC.makeimage(type='corrected', # Make an image using the
image='3c286cal1.im'); # CORRECTED_DATA column
# in the MS. Write the image
# 3c286cal1.im to disk.
The resulting image, 3c286cal.im, written to disk, has 4 planes, with
I, Q, U, and V images. Use the viewer to obtain statistics on
each plane of the IQUV image (see the Image Analysis section of this
document for details about Viewer operation). Very briefly, bring up
the image with the viewer by left-clicking on the wrench icon to
the right of the image name in the GUI input, and choosing the option
View. Once the viewer window comes up, use the arrow
buttons on the right of the viewer to display the Q image (2nd
image plane). Then, left-click on the Tools button at the top
of the viewer and choose the ImageStatistics option to
bring up an Image Statistics GUI. Click on the Statistics
option in this GUI. To obtain statistics in the 2nd image plane,
left-click on the Plane button.
Record the maximum OR minimum value, which ever has the largest
absolute value. Use the viewer arrow buttons to display the U image
and repeat this procedure. Now, using the Stokes I value reported by the
Imager.setjy function run earlier and assuming V=0, the final
polarization model is:
I = 7.462 Q = 0.4982 U = -0.6988 V = 0
If your data set has two spectral windows, create an image with spwid=2 and repeat this procedure to find I, Q, and U.
Now, set the flux density model of 3C 286 with imager.setjy and
proceed to the calibrator tool to solve for the polarization
``leakage'' terms (aka D-terms):
stokes := [7.462, 0.4982, -0.6988, 0.0] # Define polarization model
# determined from imaging
imgrC.setjy(fieldid=11, # Set model for IQUV
fluxdensity=stokes);
calC.setdata(msselect='FIELD_ID==11'); # Select data for polarization
# calibrator, 3C286
calC.reset(); # Reset setapply/setsolve
calC.setapply(type='P', # Arrange to apply parallactic angle
t=5.0); # correction
calC.setapply(type='G', # Arrange to apply flux-scaled
t=0.0, # G solutions
table='ap366.fluxcal');
calC.setsolve(type='D', # Arrange to solve for D over a long
t=86400.0, # time scale, average data within
preavg=600.0, # the solution to no more than 600
table='ap366.dcal'); # sec per chunk.
# Write the solutions to the table
# ap366.dcal located on disk.
calC.state(); # Review setapply/setsolve settings
calC.solve(); # Solve
calC.plotcal(plottype="DRI" , # Examine solutions
tablename="ap366.dcal");
The calibrator.state and calibrator.solve functions will produce the logger messages:
The following calibration components will be applied:
G table=ap366.fluxcal t=0 select=[]
P table=<pre-computed> t=5 select=[]
The following calibration components will be solved for:
D table=ap366.dcal t=86400 preavg=600 phaseonly=F refant=4 append=F
Solving for D
D Jones Slot=1, 1328+307, spw=1: 23-May-1998/00:53:15 to 23-May-1998/04:45:40
D Jones Initial fit per unit weight = 0.191679 Jy, sum of weights = 25290
D Jones Final fit per unit weight = 0.13899 Jy after 8 iterations
Storing D matrix in table ap366.dcal
The D-term solutions (Fig. 1.16) are plotted with PLOTTYPE=DRI: D-term real versus imaginary components. Most
solutions are less than 1% except for a few antennas with about 3%
leakage. This is a reasonable solution. See:
http://www.aoc.nrao.edu/
gtaylor/calman/polcal.html
for a detailed description on how to plan for good polarization
calibration and what to expect when solving for D-terms.
Now, correct the data for P, G and D as follows:
calC.setdata(msselect='FIELD_ID IN [9:11]'); # Select fields 9, 10, & 11 to
# which calibration will be applied:
# 9 = gain calibrator 0917+642
# 10 = target source 0957+561
# 11 = polarization cal 3C 286
calC.reset(); # Reset setapply/setsolve
calC.setapply(type='P', # Arrange to apply parallactic
t=5.0); # angle correction
calC.setapply(type='G', # Arrange to apply flux-scaled
t=0.0, # G solutions
table='ap366.fluxcal');
calC.setapply(type='D', # Arrange to apply D solutions
t=0.0,
table='ap366.dcal');
calC.state(); # review setapply settings
calC.correct() # Write the corrected the data
# to the CORRECTED_DATA column
# in the MS
The calC.state and calC.correct functions will report in the logger window:
The following calibration components will be applied:
D table=ap366.dcal t=0 select=[]
G table=ap366.fluxcal t=0 select=[]
P table=<pre-computed> t=5 select=[]
The following calibration components will be solved for:
None.
Initializing non-solvable parallactic angle (P-matrix)
For interval of 5 seconds, found 213 slots
Applying G table from ap366.fluxcal
Applying D table from ap366.dcal
Polarization calibration is almost complete at this point - the final step, calibrating the electric vector position angle, will be done during the imaging steps below.