[evlatests] Some Good WIDAR news

[George Moellenbrock]

Folks-

FYI,

On Thu, 30 Jul 2009, Rick Perley wrote:

>    The full 100 MHz image has an impressive peak/noise of 218,000:1,

I've achieved essentially the same dynamic range processing this data 
entirely in CASA, for roughly the same editing/calibration:  I used 85 MHz 
bandwidth, 10s bandpass timescale in the selfcal, and reached ~200-230k:1 
depending upon just where I measured the rms in the upper left-hand corner 
(of the 180 deg rotated image).

And yes, it gets nearly an order of magnitude worse in the middle.  Now 
that the bandpass is so much more carefully treated (to 10s!), these 
remaining closure errors could originate in the D-term contribution to the 
RR correlation which is made quite interesting by virtue of the 11% 
polarization of 3C286.  As I noted in the WIDAR PTC analyses last fall, 
even the constant (non-source-pol) closure-violating contribution of the 
instrumental polarization (D^2 term, where D is a few %) should be kicking 
in by 30-40k:1 (for 351 baselines).  The contribution from the source-poln 
terms vary with parallactic angle, but are larger (for 3C286) 
instantaneously.

-George
















> using a far corner to estimate the noise.  The 'noise' near the 13,000
> mJy point source is a few times larger, and is clearly due to slowly
> changing errors which are not being calibrated out.  I have not
> attempted a closure calibration  yet.
>    A single 10 MHz BW image was made -- the error patterns are the same
> (indeed, each of the 10 channels shows the same error pattern).  The
> far-out noise is larger by a factor of 2.5, rather than 3 as expected --
> meaning the noise is not decreasing as sqrt(B).  I checked the noise in
> each of the 10 individual pseudo-continuum images -- except for the
> first (RFI), the center 7 'channels' have the same noise (0.16 mJy).
> Channels 2 and 10 are 0.19.  Channel 1 is 0.26 (presumably due to
> left-over RFI).
>
>    Bottom Line:  The images are very good.  There is no sign of serious
> errors.
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