[evlatests] A curious focus problem from Ku to K bands

[Ken Sowinski]

On Mon, 9 Jan 2012, Rick Perley wrote:

>    In my recent tests, I've noted that when we change frequency from Ku
> to K bands, there are five antennas which  *always* take longer for the
> unflagged data stream to start up.  Specifically:
>    ea02 takes 10 seconds longer to give good data than most others,
>    ea08 takes 5 seconds longer,
>    ea03, 04 and (less regularly) 13 all take 3 seconds longer than all
> the others.
>    The effect is seen in all four IFs.  It's not a correlator issue --
> the data are present, but are correctly flagged -- the amplitudes during
> these intervals are low, but increasing.  So it has to be focus,
> pointing, or a slow subreflector.  This last one is unlikely, as this
> tardiness only occurs with the transition from Ku to K -- a very short
> travel for the subreflector.
>
>    Reviewing the flagging table gives the answer -- it's the focus.  Is
> there that large a difference in focus position between these two bands
> for those particular antennas?

All the antennas listed except for ea13 do not have Ku band
receivers, and therefore no Ku band focus setting in the
parameters database.   If the K band focus is much different
from zero, then it will take a few seconds to get to the
correct position when switching from Ku to K band.  The
fix is to insert focus setting for Ku band even for those
antennas which still lack receivers.