FW: FW: [mmaimcal] Correlator reconfiguration time (fwd)

[Mark Holdaway]

Larry writes:
> Harvey Liszt writes:
>  > Does this impact the decision about whether to put on a nutating
>  > secondary?  No point in swinging that back and forth if to no effect!
> 
> I think not.  It is hard to imagine any mechanical beam switching at a
> rate faster than a few Hz.  None of the factors limiting the dump rate
> (phase switching, hardware transfer rates, signal processing
> capability) is expected to be in effect at this rate.  Besides, beam
> switching like this is only for single-dish mode, where only the
> autocorrelations are needed and where phase switching has no effect,
> thus easing the limits considerably.
> 

This addresses just a tangential point of Larry's: maximum switching
frequency.  But I'm a tangential kind of guy.

I believe the SMT has many switching modes (Jeff Mangum can tell you the
details), and the fastest is something like 10 Hz, although that one
doesn't throw very far.  I can't remember if I thought about or worked on
the idea of designing the beam switching cycle for optimum SNR in the
presence of a model atmosphere with inhomegeous water vapor... but it is
clear to me that these cycles are generally not designed optimally.. ie, a
designer or engineer assumes its best to get a large fraction (ie, 90%) of
the time on the ON and OFF positions: but if you are atmosphere limited,
it may actually be better to trade thermal noise for speed, spend a
smaller fraction on source and switch faster.  (Beam throw is another
parameter that needs to be traded off with as well.)  So, higher switching
frequencies are likely possible without changing anything fundamental
about the nutator design, just the design of the cycle.

	-Mark