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

[Ken Sowinski]

On Fri, 6 Oct 2006, Rick Perley wrote:

> 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.

This is a troubling observation.  The phases are reported
referenced to a VLA antenna so it is hard to see that the
Flkes can do this.  Similarly it is hard to see that the
system controller can do this.  Making it break indiviual
antennas or IF pairs is very hard to do.  It is even harder
to have it modify phases in a systematic way over long periods.
It has no idea whether an antenna is an EVLA or VLA antenna.

This report ought to be compared with the VLA broken
antenna list to see if Rick is rediscovering known VLA
LO problems.