[evlatests] Friday evening tests -- Some Good News!

[Rick Perley]

    As noted by Ken yesterday, we decided to observe a strong calibrator 
with known structure -- 3C273 -- for 1.5 hours in 'fast continuum' 
mode.  The primary goal was to try learn more about our dropout problem, 
but also to enable imaging tests, as we had all five EVLA antennas 
available and operating at X-band.  We observed in 3 scans of 30 minutes 
each. 
As there seems a good chance the drop problem is now understood, Ken did 
not interrupt any of these scans. 

    A)  Stability

    All 5 EVLA antennas operated, although as noted below, 24C is not in 
good shape (as also noted by Ken). 
It should be noted that we had 27 operating antennas!!!

    Antenna 13:  All four IFs have the 10-second drop, on all three 
scans.  The drop is by 0.76 in amplitude for all scans
and IFs.  Phase behavior was as normal for the band, configuration, and 
weather (a dark and stormy evening). 

    Antenna 14:  Amplitude stability is outstanding for IFs A and C.  
Better even than the best VLA antennas.  A slight
perturbation in gains were seen coincident with the passage of a hot 
cloud through the beam (as clearly seen in Tsys). 
But the perturbation is small compared to most VLA antennas!  On IFs B 
and D, a few 'gain spikes' were seen -- very short duration and quite 
rare.  These are not matched by Tsys variations, so something else has 
diddled the correlator
coefficient.   Note that the overall stability was maintained, even as 
the Tsys dropped from about 85K at the beginning
to about 40K at the end (as the receiver cooled down).  This is good 
news -- the system is working exactly as it should. 
    Phase behavior was as expected. 

    Antenna 16:  The gain is wandering by significant amounts, both up 
and down, on long (tens of minutes) timescales. 
All IFs are doing it identically.  Review of Tsys shows similar 
variations.  My speculation is that the Tsys is being erroneously 
measured, and these values are then imposed (post-correlation) onto the 
(proper) correlator coefficients. 
    Phase behavior as expected. 

    Antenna 18:  IFs A and C are outstanding.  Just like antenna 14.  
IFs B and D have 'double drops', by 0.66 and 0.26.
Nothing is different here than has been reported before. 
    Phase behavior is as expected -- excellent. 

    Antenna 24:  Very interesting ...  IF A has drops, by factor 0.76, 
for the first 30-minute scan only.  IF C has the same drop behavior, but 
the sensitivity of this IF is appallingly bad -- as expected from Ken's 
report.  However, a sudden change in sensitivity occurred about 60% of 
the way through the run (but not associated with scan change) -- the 
amplitudes rose, and the sensitivity improved (but no where near to what 
is normal). 
    Phase behavior is fine!  There are no significant long-term drifts, 
so the X-Y baseline components for the master pad are pretty good.  (I 
can't measured Z component with a single source). 

    Once the drops were edited out (very easily done with the AIPS 
super-program EDITA), the amplitude stability of the 'dropping antennas' 
was outstanding (other than 24C, which clearly has other problems). 
    For all EVLA antennas, phase stability was excellent, with no 
significant longterm drifts, and short-term behavior clearly caused by 
weather. 

    B)  Imaging. 

    As noted above, the source was selected because of its well known 
structure and strong point-source, affording us the possibility to look 
at imaging and closure issues.

    As the observations were taken in 50 MHz continuum, signficant 
amplitude loss was expected on EVLA-VLA baselines.  This was seen, of 
course.  Standard calibration was done, resulting in large apparent 
'excess amplitude' seen on EVLA-EVLA baselines. 
    A blind image, using all antennas, was made,  which was 'pretty 
good', but showed the 'closure' errors as a lumpy
north-south stripe of residuals (these show up this way because the 
source declination is only +2 degrees). 

    A much better image, using the VLA-VLA baselines only, was then used 
to determine the baseline-based 'closure'
corrections.  A single solution, for the entire 1.5 hours, was made 
using BLCAL.    The typical VLA antenna corrections are 0.999 to 1.001 
in real, +/- .001 in imaginary.  The corrections for EVLA - EVLA 
baselines are 0.95 in real, +/- .005 in imaginary.  VLA - EVLA baseline 
corrections looked the same as VLA-VLA baselines -- as expected. 

    The result of applying these corrections was spectacular!  The image 
using all 27 antennas showed no discernible
N-S 'closure' pattern, and rms noise of 0.36 mJy -- a dynamic range of 
70,000.  Not bad for 1.5 hours continuum data.
For comparison, an image using only the VLA antennas was made -- the 
noise level was the same, as was the dynamic range. 

    I will point out here that the new capability of  IMAGR to specify 
antennas to be included in a map was a very great help to this study.  
Thanks Eric! 

    The excellent image made with all antennas clearly demonstrates that 
the EVLA 'closure' error is stable, and can be corrected for with very 
high accuracy.    Very good news indeed (but not surprising to me).   
Users should not be dissuaded from wideband continuum observations -- 
but they need to be informed about how to do the extra calibration.  
Fortunately, BLCAL is a very easy program to run. 

    Some disappointment may accompany the revelation that the noise 
level did not improve with the addition of the
5 EVLA antennas.  This is actually no surprise.  We thermal noise level 
is well beneath the observed noise in the map.  These are due to 
residual closure errors, likely small time variations which are on all 
baselines.  I also made to heroic efforts to flag bad data, or execute 
any of the tricks we've learned to identify discrepant observations. 

    C)  EVLA antenna sensitivity. 

    I computed the AIPS weights, to judge antenna sensitivity.  The 
results are mixed:

    Antenna 13 is good on all IFs.  Just like a VLA antenna -- as 
expected. 
    Antenna 14 was very bad at the beginning, but steadily improved, 
reaching normal (like VLA) status at the end.  This behavior was 
expected, as the receiver cooled down from about 85K at the begnning to 
~40K at the end. 
    Antenna 16 is just awful, the worst in the array (save 24C -- a 
special case).   As noted before, Tsys is varying all the time, and is 
high -- 100 to 140K.  There is evidence that the Tsys variations are not 
'real' -- in the sense that the correlation coefficients may show no 
variations.  If Tsys was really changing, the raw correlation 
coefficients would reflect the changing noise.   (I say 'may' as I don't 
have the raw values, nor have I gone to the trouble (yet) to 
cross-correlate the gain solutions with the Tsys values). 
    Antenna 18 is good on all IFs -- about normal. 
    Antenna 24 is very poor on IF A, and a total disaster on IF C.  The 
former problem is likely due to a very high Tsys (about 125K).  The 
latter cannot be blamed solely on the 100K system temperature...