[mmaimcal] antennas

[Jeff Mangum]

"Al" == Al Wootten <awootten at nrao.edu> writes:

...
Al>  As currently practiced, offset pointing is done semi-hourly or so and so
Al>  can't entirely work against wind buffeting.  I presume that you are considering
Al>  an offset pointing done on the cal at each cycle of fast switching.  We
Al> discussed this at the MAC meeting.  Someone--Jack Welch?--suggested that it
Al> would only take a few seconds to accomplish.  However, this
Al>  will limit the fast switching duty cycle it seems to me.  We need to model
Al>  fast switching with pointing included in the cycles, which we have not done.
Al>  This might take out some pointing jitter on sub-minute time scales.  However,
Al>  I thought Mark's memo showed that at lower elevations, where the cross section
Al>  of the antenna to the wind is largest, we were already pressed to get enough
Al>  time on the source during the duty cycles he investigated; trying to put
Al>  pointing into the duty cycle also can only make things worse.  I think
Al>  that too heavy a dependence on offset pointing, especially to take out wind,
Al>  might mean a heavy penalty in terms of science.

I guess I should pass-on some information about the 12m pointing
behavior.  The basic pointing model is very stable, correcting for the
large-scale, time-independent motions of the structure.  Under good
conditions (no wind, stable temperature, no sun on dish), this model
need be checked only every few hours.  If one has windy conditions, or
if the dish is being differentially heated in any way, then the
pointing must be checked much more frequently.  It quickly becomes
impractical to try to correct for variabilities which happen over
timescales less than about 1/2 hour, so how do you deal with those
sorts of variations in your structure.  A good solution is to have a
laser metrology system.  We are prototyping such a system on the 12m
for monitoring the motions of the prime focus (this system has existed
on the telescope for nearly 10 years, but due to manpower limitations
has not been implemented).  With such a real-time system one can
watch the telescope bend and twist.  Once this bending and twisting
can be understood, then the pointing model can be enhanced to
incorporate this information (which is taken in real-time).  So, I
think that there are ways to deal with the short time scale variations
of the structure which affect the pointing characteristics of the
antenna, but this is still a development project, although a promising
one.

Cheers,

Jeff