[evlatests] Deep Imaging!

[Rick Perley]

    Some other evidence, favoring George's hypothesis are:

    a) The required closure corrections for this recent 6cm database are 
of order 0.3% -- the same level as noted in my previous deep 6cm images, 
done in different configurations. 

    b) The closure corrections required for 3.6cm observations are much 
lower -- typically .03%.   

    The C-band polarizers are considerably 'leakier' than those at 
X-band -- the ratio being a factor of a few, as required by the 
factor-of-ten  difference in the closure corrections. 

    We should determine if the closure corrections determined on one 
source can be transferred to another -- if both sources have  negligible 
polarization (compared to the D-term amplitude), this should be effective. 

   I am not copying over the 66 GB L-band observation of 3C147.  This 
object has a linear polarization of less than 0.01%, and is completely 
unresolved.   If George's hypothesis is right, a baseline calibration 
should really be effective in improving the dynamic range.  Stand by ...

   

George Moellenbrock wrote:
> Rick, et al.,
>
>   
>>    The BLCAL solutions were generated for the entire 6-hour duration --
>> one solution per baseline.  It would seem that this is all that is
>> needed to reach thermal noise at this level.
>>     
>
> A bit dishonestly, though it is gratifying that a constant correction
> was effective.  Alas, this is not a general solution.
>
> So, what is the origin?
>
> D-terms.  For finite D-terms that differ among the antennas, dynamic range 
> will be limited to something like 1/(<D*D>*sqrt(N)) where <D*D> is the 
> mean square of the typical residual D-term (that part not partially 
> absorbed by the gain calibration), and N is the number of baselines.  For 
> D-terms of a few percent and 12 antennas, a few 10000s:1 may not be 
> surprising at all.  (Rick just stopped by and noted that the C-band 
> polarizers are often worse than a few percent.  At X-band, where imaging 
> works better, the Ds are known to be better than at C-band.)
>
> Rick has also pointed out that the sources he images are weakly polarized. 
> Thus the D-term contribution to the closure errors will be ~constant 
> (especially if the Ds are large), consistent with constant BLCAL 
> solutions.
>
> We have 3C48/3C84 data in hand to test the hypothesis that a polarized 
> source (3C48) requires variable BLCAL.  I went on vacation and haven't
> reduced it yet.  (Steve has such evidence from 3C286, in fact.)
>
> The upshot is that the newly available sensitivity and the fact of 
> bandpass-calibrated continuum (and no variable ripples, etc.) now expose a 
> non-closing feature that has always been there.  Alas, there's no free 
> lunch.
>
> -George
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>   
>>    Two baselines for 0555+398 were seen to have high residual
>> amplitudes or phases (probably oscillatory).  I'll be looking into these
>> now.
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>>     
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