[evlatests] antennas 18 and 24, and a curiousity

[Ken Sowinski]

Some dynamic time was used this morning to track down K band on
antenna 18 and the flighty delays for 24A that Rick and I have
been discussing.

K band was made to work by pointing at the water maser in Orion
and moving the subreflector around till the maser was seen in
autocorrelation.  It turns out that as I wrote a while ago, the
difference in rotation setting for K and Q bands was about right;
what was wrong was the sign.  The rotation was misset by over
40 degrees.

Having found that I was able to set delays and focus and tune the
rotation for best response.  At leisure I found the error in the
surveyed subreflector settings.  The "optimal" setting I found is
about a degree away from the surveyed position.  That is not
important at K band, but will nonetheless be fixed next week and
the collimations adjusted to accomodate the change.  Antenna 18 is
now usable at K band.


I examined cross and auto correlations of antenna 24 to try to
see what is wrong with the delays.  At a BW of 12.5 MHz the spectrum
looked very funny.  I tried resetting the deformatter in the DTS
module.  That made a difference but time had run out before I
could figure out what it meant.  I am convinced that the poor
behavior of both IFs in 24 is due to the DTS modules.


The curiousity involves antenna 13.  Initially the antennas were
all observing at K band in SYSSTART and antenna 13 was reporting
an erroneously low SD voltage, therefore falsely boosting the 
amplitudes.  Half an hour later the SD voltages were normal again
with no intervention on my part.  During this interval TP, ALC
and sampler statistics all looked normal.  The newly installed
downconverter in 13A has power detectors.  The standard deviation
of the detector signals is a good proxy for SD.  The RF detector
RMS looked like what I saw yesterday at K band; the output detector
RMS was much smaller than expected.  Perhaps something was not
setup properly in the downconverter for the first few scans.