[evlatests] C/X Phase test, Oct 3/4.

[Rick Perley]

  Some dynamic time was available on short notice, so I made up a 3.5 
hour observation of a strong calibrator, 3c454.3, in which I observed 
alternately between 6 and 3.6 cm, with two minutes on each.  The 
observation began near midnight, at meridian transit .  At the 
beginning, the elevation was 71 degrees.  At the end, it was 39 
degrees.  The weather was calm and dry,  with T = 7.5 C (+/- 1.0), DP = 
0.7C, and wind of 1 m/sec (+/- 1msec).    All observations were made at 
the default frequencies, with 12.5 MHz BW -- hence the Flukes are set to 
their default value of 118.75/218.75 MHz. 

    Oddities were found both for EVLA antennas, and VLA antennas.  I 
report on these separately.

A)  EVLA antennas. 

    a) There were no phase jumps or other short-term phase oddities 
between EVLA antennas. 
    b) When referenced to a stable VLA antenna, three of the 41 
individual scans had a large phase jump.  The jumps are all identical 
for each EVLA antenna/IF.  For two of these, the jump was the same. 
    c) There were a number of minor amplitude issues -- usually at the 
beginning of the scans -- which I mainly attribute to the reaction of 
the Tsys monitoring machinery reacting to changes in power when changing 
bands.  
    d) There were notable long-term phase drifts for all EVLA antennas.  
I used a central VLA antenna as phase reference (against which all VLA 
antennas are stable -- however see the important notes in section B for 
exceptions).   The table below shows the total phase change over 3.5 
hours for each EVLA antenna/IF, at each band. 

Ant.        IF         C-phase         X-phase
-----------------------------------------------------
13         AC           +40                 -140
13         BD              0                 -180
14         AC           -10                   -90
14         BD           -60                 -130
16         AC           110             no fringes
16         BD            70                 -270
18         AC              0                  -50
18         BD           -50                  -90
24         AC          +20                  -70
24         BD           -20                 -110
26         AC         no rcvr           +750
---------------------------------------------------
   
    Observations:
       i) The huge phase wind for antenna 26 is likely a baseline error 
-- this corresponds to about a 10 cm error, larger than I thought we 
had, but plausible I suppose.  Ant 26 is now on a different pad, so this 
can be easily checked with fresh data.
      ii) The BD phase change is always about 40 degrees more negative 
than the AC phase change. 
     iii)  The phase change is smooth in all cases, but is certainly not 
due to baseline error.  If due to uncompensated path length, I would 
argue that it is internal to the antennas, since the outside temperature 
was constant, and the data were taken mostly past midnight, certainly 
long enough for equilibrium to be established.  But if it is due to 
internal temperature changes, why should these be changing during a calm 
stable night?  Perhaps more likely is an elevation effect (which might 
combine internal temperature stratification)?  But why would 13BD show 
no effect at all at C-band, but a half-turn of phase at X-band?  Indeed, 
the differences between 6 and 3.6 cm behavior are puzzling to me.  
Perhaps a true pundit can explain this. 

B)  VLA antennas. 

    A new and very troublesome phenomenon was immediately found -- Some 
antenna/IFs have non-repeating phases (after having changed band). 

    The effect was especially noted at X-band, but was present in C-band 
as well.   I emphasize that the EVLA antennas did not see any of the 
phenomena described below. 

    At C-band, antenna/IFs 12AC and 20AC showed multiple phase states -- 
the phases are stable within each 2-minute observation, but the observed 
phases were as much as 80 degrees away from the 'base' state.   This 
effect was especially bad in these two antennas, but 'single' versions 
of this occurred in two others:  15AC, 25BD,   In addition, phase 
'spins' occured in some scans in many other VLA antennas:  3, 7AC, 9AC, 
10AC, and 11AC. 

    At X-band, the situation is worse.  3BD has a bistable phase, with 
phase spins between them.  Multiple (but mostly stable within a scan) 
phases are seen on 5AC, 6BD, 8BD, 9AC, 12AC, 20AC, and 27AC. 

    The consequence of these unsteady phases upon VLA imaging is likely 
to be significant. 

    I was ready to blame the Flukes for all this, but Ken is wondering 
if the new system controller is responsible.