[evlatests] Some Good News!

[Rick Perley]

    Michael took an hour of data on our favorite northern naked source, 
0217+734. 

    Differences from former tests are (so far as I know)...

    1) X-band, rather than C-band.
    2) 2 IFs, (sub-bands) rather than 4.
    3) Point source was offset from phase center by a small amount 
(about 0.3 arcseconds, I think). 

    There is a world of difference between this database, and all the 
others taken this month ...

    1) There are *NO* 180 degree phase jumps.  All data are completely 
seamless in amplitude and phase. 
    2) Except for the initial couple of minutes, there are no bad data.  
I cannot recall ever seeing such lovely data ...
    3) Histograms of the single-channel distributions (both real and 
imaginary) show absolutely perfect gaussians, with 1-sigma widths of 
0.655 Jy -- a bit better than what I expected. 
    3) Initial images showed our 'lumpy-bumpy' problem.   But read on ...

    To uncover the origin of these lumps/bumps, I SPLITed the data into 
a single 961-channel average (to increase SNR) and plotted the amplitude 
and phases for all baselines and times.  The amplitudes looked just 
fine, but the phases showed a single baseline with an oscillatory 
pattern which persisted throughout the entire hour of observation.  The 
pattern frequency decreased throughout, reaching a zero at about 14:50 
IAT, at a U-spacing of -2.1 Klambda -- i.e., this problem has nothing to 
do with a steady external stationary signal (which must maximize when 
the source fringe rate goes to zero -- at U = 0). 
      
       The evil baseline is 2 x 18.  The amplitudes look fine, but the 
phases are oscillating by +/- 2.5 degrees.  Both IFs (sub-bands) behave 
identically (except that IF#2's phase oscillations are inverted w.r.t. 
#1). 

    I then flagged the evil baseline, and re-calibrated.  BLCAL was run 
(single solution for entire hour).  The baseline corrections are 
*extraordinarily* small -- typically 1 part in 10^5, consistent with 
zero, I believe.

    The single-IF (#1) map which resulted from this is almost perfect.  
The rms (it doesn't matter where it is measured, the background is 
completely flat) is 55 microJy -- this is *exactly* what it should be 
based on the single-channel histogram width reduced by the square root 
of the number of points (150602) and channels (916).  The resulting 
formal DR is 78200. 

    There is a barely detectable waviness seen beneath the noise.  As 
this is hardly attenuated at all throughout the image, it appears these 
could be a very few unflagged points which escaped by (almost 
non-existent) flagging. 

    So!  Why is this X-band test so good, while the preceding C-band 
ones so bad?

    And -- we still have a single very bad baseline.  What is wrong with 
it? 

    Finally -- other than that bad baseline, the corrrelator looks to me 
to be behaving precisely right. 

    I'll do more checks tomorrow.  It's Sunday morning, and my garden is 
calling me home ...