VLA Test/Observation Coordination Meeting
                           B.G. Clark
                        December 20, 2001


1.  K band behavior

C. Chandler reports that the new K band receivers have occasionally 
had bad system temperatures, in episodes lasting a few days, especially
in summer.  This has been traced to the condensation of water in the
feeds.  It turns out that the feeds are not very well sealed.  There 
are many joints in the feed, some of which have not been installed
with O rings.  Although the feeds are purged when the receiver is 
installed, and a desiccant is attached to the feed breather hole, 
apparently enough air is leaking through the joints to promote condensation.
The front end group is experimenting with tightening up the feed, with
O rings in every joint.  Three feeds have been done as a trial, and 
one of those showed condensation.  Taking the feed apart and installing
the O rings is a pretty big job, and is likely to take several months.
(O rings are on order, not yet received.)

CC updates the current status, noting that there are 17 new K band 
receivers installed on the array.  The new receivers cover from 20.6 GHz
to 25.6 GHz, although there are few local oscillator system that do not
lock up at the ends of this band.

2.  Calibrator flux ratios

R. Perley reports on the analysis of the latest one day run of the 
calibrator flux ratio determination program.  The weather was very good,
and the system was extremely stable.  The internal RMS of the flux
ratios as a function of band is given below.  (L band is omitted from
the table, because the scatter is more likely determined by confusing
sources than by instrumental stability.  Confusion may also be a detectable
effect at 6cm.)

	  6cm    3.6cm     2cm     1.3cm      0.7cm
SD      0.002    0.002    0.004    0.008      0.030

The numbers in the table above are about what one would expect from
a 9" rms pointing error, which may indeed be the most significant
contributor.

Antenna 1 behaves most curiously.  The 1.3cm and 2cm gain curves
peak at different elevations, and the 0.7cm gain curve has a very
large effect (more than 20 %), and measurements at 0.7cm scatter
by many percent.  It clearly has a problem with its optics, and 
possibly with its pointing as well.

NGC 7027, which should be entirely unpolarized, at 3.6cm shows formal
polarization solution (q,u,v) = (1 mJy, 2 mJy, 3 mJy), out of a
map continuum peak of 4 Jy.

The VLA flux scale is essentially based on the Baars et al. formula for
3C 295.  The calibrator flux ratio runs have indicated that the 3C 286
to 3C 295 ratio is unchanging over the years; all other calibrators 
appear to be variable.

The Baars scale doesn't really extend to 0.7cm wavelength.  Extension
can be made in two ways.  First, the planetary nebula NGC 7027 is believed
to be entirely thermal radiation from optically thin ionized gas.  
Second, good models exist for the surface of Mars, with temperatures
determined from actual thermometers, and well known dielectric constants
for estimating emissivities.  These two procedures seem to result in
disagreements at about the 5% level in the flux scale.

3.  Pointing

C. Carilli reports that the new elevation encoder electronics which are
installed on antenna 15 do seem to be working well after their initial
adjustment.  Once the gain and offset terms were set, there is no sign
of a periodic error arising in the electronics.  The encoder electronics
will be replaced on the rest of the array over the course of the next 
year or thereabouts.