[evlatests] Antennas 13, 14, 16 and 18 at X-band

[Rick Perley]

    I used my available time to do deep integrations at X-band, where we 
have four EVLA antennas, on a strong point source, 3C84, to monitor 
stability and closure.   Having four antennas is important, as this 
allows a solution independent of the VLA antennas, and allows an 
independent estimate of non-closing errors -- gain fluctuations which 
are not factorable into antenna gains. 
    Unhappily, the Friday time (3 hours) had only 3 EVLA antennas 
working, due to the massive power failure.  And the Saturday time (6 
hours!) had only 3 EVLA antennas working due to antenna 18 having an 
inoperative L302. 
    Ken was generous enough to give me two hours of his software time 
today, and for this we had four working EVLA antennas.   Data were taken 
in IFs A and C, parallel hands only, with 64 channels per corrrelation, 
6.25 MHz BW, giving a resolution of 97 kHz.  This was sufficient to 
resolve out the sharp baseband filter edge, nicely eliminating the 
'Gibbs Ringing' which figured prominently in earlier messages. 
Averaging was at 5 seconds. 

    Results:

    1) Stability. 

    No drops and phase oddities were seen, except that all four EVLA 
antennas jumped 150 degrees in phase
(w.r.t the VLA antenna phases) during an 8-minute gap in my observations 
which occurred around 19:35 IAT.  I didn't get a log from the operator, 
but received a message about this time saying something was changed.  
Barry was at the site of the time, perhaps he can provide some insight 
here.  Otherwise, phase behavior was normal. 

    Amplitude stability varies:

    For both today's run, and Saturday's long run, antenna 13's 
amplitude stability was fantastic, with pk-pk variations of 0.5%, and no 
trends visible.  Indeed, the stability is so much better than *any* 
other antenna, EVLA or VLA, that I'm actually worried, although I'm not 
sure what I should worry about ... 
    Antennas 16 and 18 are also very stable, but the overall gain 
dropped slowly throughout the two hours, but ~3% and 1% respectively. 
    Antenna 14 is unstable at the 2 to 4% level, with sudden changes in 
gain ocurring for now obvious reason.  Some of these are reflected in 
Tsys changes, most are not.  Something not quite right is going on here. 
    In general, the VLA antennas showed many gain peculiar changes, 
especially on Saturday, where amplitude drops up to 10% (!) were noted 
on some antennas.  The weather was dry and breezy, but we cannot account 
for these changes by invoking a wind.     All VLA antennas showed odd 
(and slow) drops, with no visible correlation between them.  No 
corresponding effects in Tsys were noted.  The array was acting like 
there was some cold absorber up there, which mysteriously avoided 
antenna 13!   I personally think there's a more mundane explanation, but 
can't think of one offhand. 

    Bandpass stability was excellent on all EVLA antennas.  At least as 
good as VLA antennas. 

    Sensitivity:  The EVLA antennas, except for 14C, are better than 
nearly all VLA antennas.  Antenna
14C is notably worse, and its Tsys value is 50% too high.  Sounds like a 
troubled receiver. 

    Closure.  Some worries here.  The EVLA antennas show closure at the 
1.5% level, in amplitude.  They do this with the VLA antennas, and with 
each other when separated out.  Although it's possible this is due to 
real structure, I think it unlikely, as similar VLA baselines don't show 
excessive or deficient amplitudes.  However, there are similar sized 
closure errors on VLA-only baselines.  We know 3C84 is very slightly 
resolved at this resolution, but the sharpness in the resulting 
visibility changes argue that the closure, if due to structure, is not 
related to the source itself, but would originate from a large angle.  
Unlikely, in my view.  But not impossible. 

    Chasing the closure issue would require much longer integrations on 
very simple sources.  This will have to wait.  Better ideas will be 
gratefully entertained.