[mmaimcal] Vertex Antenna turn around time

[Jeff Mangum]

Hi Folks,

I have been looking into this issue of the current Vertex design
performance for maximum slew rate turn around time.  Recall that the
current design indicates a turn around time of 0.8 seconds assuming a
30'/sec scan rate and a scan time of >2 seconds in OTF mode.  Note
that this implies a >1 degree scan length at the maximum rate.  I have
reviewed this issue with Mark Holdaway, where the basis for the
discussion relied on Mark's unpublished memo on a comparison of beam
switching and OTF for continuum total power measurements.  From that
work, Mark found that OTF total power observing is as good as or
better than beam switching for optimal sky subtraction when one could
OTF map at the maximum slew rate (1 deg/s) and a 0.2 second turn
around time.  So, Vertex is within a factor of 8 of meeting Mark's
simulated break-even point.

If we assume that all antennas will have nutators, then the current
performance will not be a problem.  Note also that we will not be able
to resolve this issue of whether fast OTF gives us the same capability
for sky subtraction as nutators on the prototype antennas, so if we
ask Vertex to do better, we have no way of testing whether this was
necessary.  So, given all of this, I am inclined to say that the
current fast motion performance is ok.

Any comments?

Cheers,

Jeff