VLA Test/Observation Coordination Meeting
                           B.G. Clark
                         June 16, 2005

1.  EVLA

Some, and perhaps all, of the problems we have been seeing with sensitivity
and aliasing are now understood to be a problem with the frame alignment
system of the Data Transmission System.  A new digitizer module, with
a new ADC and different alignment logic is now in hand, possibly ready
for testing today.

2.  Polarization stability

G. Moellenbrock has accumulated many sessions of polarization measurements
at L band to evaluate stability of polarization correction, and its 
frequency dependence.  He started observing with 7 channels, 25 MHz
bandwidth, and then went to 15 channels (to resolve the 3MHz waveguide
induced ripple).  He uses multiple LO settings to cover the entire
1250-1750 band.  He observes 3C 84, whose intrinsic polarization is very
much smaller than the D terms, and can be neglected.  The overall picture
is that the instrumental polarization is good in the center of the band,
and rises toward 1300 MHz especially.

After suitable calibration of the parallel hands and adjustment of the
phase of the R-L phase difference, the curves of polarization vs frequency
overlay very well indeed on each other.  One date is seriously different
from the rest.  It is suspected that this arises from snow or water in
the dish (the observation was in January).  GM notes that the aligning
of phases that he did to bring all the patterns at different dates 
together essentially amount to calibrating the E-Vector position angle
of the array, and, if we were sufficiently organized, this could be
used for astronomical observing, instead of having to use the highly 
polarized source 3C 286.

Two antennas have hybrid polarizers, rather than the notched septum type.
Hybrid polarizers will be used for this band in the EVLA upgrade in order
to get the required bandwidth with reasonable losses.  The crossed-
hand fringes with other VLA antennas are generally higher - this is
not unexpected, and just says that the systematics are different,
not necessarily worse.  A major change in the pattern was tracked down 
to the replacement of one of the receivers.  The crossed hand fringes 
between the two were also larger than average, but not excessively so 
in the current state of development of the receivers.

He has a couple of runs with EVLA antennas.  This is not sufficient for
evaluating the polarization stability of these antennas.  The most
obvious effect seen is the phase mismatch at the lower end of the
band.

3.  200 MHz system.

L. Greenhill reported on current status.  There are now three antennas
with 200 MHz receivers.  There is a fourth receiver, but it has returned
to Cambridge for repair and tests.  The digital TV signal in channel
nine makes it impossible to use the receivers at their intended 
frequency, but 1 MHz filters have been installed to allow them to test
and calibrate the system in an interference-free band.  They find 
a System Equivalent Flux Density of 3700 Jy.  This is slightly higher
than they were expecting, and they are looking into directors on their
dipoles to increase antenna efficiency.

The dipoles seem to have no measurable impact on the G/T of the L band
system.  If they are shorted, they act as directors for the P band feed,
and actually slightly improve the SEFD, though they do somewhat widen
the beam.

To do the science the proposers want requires observing at about 196 MHz,
which is seriously interfered with by DTV stations on Sandia Peak (channel
9) and Santa Fe (channel 10).  However, these are educational stations
with a certain sympathy for science, so prospects of coordinating turning
them off for a few hours late at night are fairly good.  There are still
other stations seen in the receiver, and they are looking at installing
crystal notch filters, up to about six per polarization, to eliminate
the carriers.

For general use, coordinating with the TV transmitters is too burdensome.
There are reasonably open bands at 174 and 216 MHz, which could be 
implemented for general observing, with some loss of G/T.

4.  Standard Field Observation.

It has been the custom in C configuration to make an observation of a
standard field, and to run tests of all useful correlator setups to make
sure we haven't lost some capability by accident.  G. Taylor points out
the we did not reduce last year's observation, because on lack of manpower,
and raised the question of whether it is worthwhile continuing to custom.