[mmaimcal] antennas

[Min Yun]

> > 
> >  As currently practiced, offset pointing is done semi-hourly or so and so
> >  can't entirely work against wind buffeting.  I presume that you are considering
> >  an offset pointing done on the cal at each cycle of fast switching.  We
> > discussed this at the MAC meeting.  Someone--Jack Welch?--suggested that it
> > would only take a few seconds to accomplish.  However, this
> >  will limit the fast switching duty cycle it seems to me.  We need to model
> >  fast switching with pointing included in the cycles, which we have not done> >  This might take out some pointing jitter on sub-minute time scales.

What these hourly pointing corrections do mostly are taking out are
large scale errors in the pointing model, including perhaps some
thermal deformations.  So, these offset pointing can help solving
this kinds of problems, but this is not going to help with high
frequency (in space and time) errors.  I estimate that the pointing
performance of the VLA is about 30% worse in practice than it 
could be simply due to not having perfect operational support
such as keepping pointing model updated in time.  Also, problems
with the encoders is the single dominant source of pointing error
for the VLA right now, and it is not corrected by offset pointing
because the error pattern has spatial scales of 1-2 degree of arc.
Perhaps these are more operation issues?




						-- Min