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<p>Herein, a short summary of the results of the polarimetric
imaging experiments. These are from a 5-hour observation of
3C286, OQ208, 3C287, and 3C273 done in January, 2019, in C
configuration. <br>
</p>
<p><b>1) Total Intensity. </b><br>
</p>
<p> All four sources show the effects of 'non-closing' errors --
errors which cannot be factored out by antenna. The achieved DRs
are 15 to 25 thousand:1 -- not very good. <br>
</p>
<p> The baseline-based solution program 'BLCHN' significantly
improves the total intensity images. The improvement factor is
related to the polarized flux, and flux density, of the source --
near perfect results for OQ208 (unpolarized) and 3C287 (3%
polarized). For 3C286 and 3C273 (both 10% polarized, or more),
clear residual remain, especially for 3C273 (which is by far the
strongest source). <br>
</p>
<p> Note that I ran BLCHN to solve for non-changing errors
(single solution over the entire duration). It is very risky
running the program with shorter time solutions, as it will
cheerfully give you back the solutions required to make your data
perfectly match the model you gave it. <br>
</p>
<p><b>2) Polarized intensity</b></p>
<p> BLCHN does nothing for Stokes 'Q' and 'U' images. These
clearly show similar imaging errors to those seen in 'I'. For
3C286, it is also clear that a dominant error is that due to a
changing R-L phase. Lesser evidence for this is seen in 3C287. <br>
</p>
<p> The program RLCAL finds and removes 'R-L' phase differences.
Application of this program made significant improvements in the
3C286 polarimetric images (dec = 30.5), small improvements to
3C287 (dec = 25.2), and no improvements for 3C273 (dec = 2). <br>
</p>
<p> The R-L corrections found by RLCAL appear to be independent
of the reference antenna used in the calibration. This is a most
puzzling result, as it is inconsistent with the R-L phases found
by differencing the parallel-hand solutions. <br>
</p>
<p><b>3) Bottom Line</b></p>
<p> My best images, following application of the best calibration
practices (including the specialized programs noted above), are
far from thermal noise. For 3C273, the final images are a factor
of 15 noisier than the expected thermal noise (in all Stokes'
parameters). (!!!). These images are far worse than those I
obtained in the 1980s using the 'old' VLA. For 3C286, the 'I'
image is about 50% noisier than thermal, the Q and U are a factor
of 4 noisier than thermal. The higher noise is seen everywhere in
the images -- not just in restricted areas (like the N-S 'stripe'
seen in the 3C273 images) -- although it is worst in these areas.
<br>
</p>
<p> The `R-L' issue has been an interesting diversion -- but it
is not the central reason for the poor results for 3C273. It's
quite clear that the R-L problem (whatever its origin) can be
avoided by not observing within ~10 degrees of the zenith. <br>
</p>
<p> I'm personally convinced that the basic problem causing the
poor imaging results for 3C273 is due to coupling of the Q and U
into I (and vice versa), and thus results from
insufficient/incomplete polarization calibration and subsequent
imaging. How best to correct this remains an open question. <br>
</p>
<p>Rick<br>
</p>
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