[evlatests] Settling Times for Referenced Pointing

[Bob Hayward]

Bob B.

And it would be nice if you could to replace the motors and/or gearboxes 
at the same time so that the telescopes could slew faster that the 20 
deg/min in elevation and 40 deg/min in azimuth that they do at the 
moment (as a comparison, the VLBA antennas move 30 and 90 deg/min 
respectively).

Remember that the original design of the VLA assumed most observations 
would require long periods of tracking on the astronomical source to get 
a good image (say 8 hours so the UV tracks from each arm would overlap 
the next). Then somebody discovered how well the VLA actually works in 
"snapshot" mode. So now the antennas are slewing all over the sky, 
jumping from target to target with numerous calibrator sources in 
between using what we would consider today to be an undersized drive 
system.

Does anybody know how much of the time the telescopes actually spend 
slewing rather than tracking? My uneducated guess would say 10-20% of 
the time, depending on the observing program. If you could speed the 
drives up by a factor of two, you could get 5-10% more time on the sky 
looking at your favorite sources. That is the same gain in sensitivity 
you would get by reducing the receiver temperature performance of every 
one of the front-ends by 1 or 2 degrees Kelvin (which is an unlikely 
scenario as they're already as good as current technology allows). Put 
another way, I think it is equivalent to the sensitivity improvement you 
would get by adding an extra 25m antenna to the array (i.e., a full time 
28th dish). So if you want to improve the sensitivity of the array, 
upgrading the drives might be a relatively cheap way to do it.

My 2 cents worth on how to keep Bob B. busy...

-Bob H.


Bob Broilo wrote:
>> A modern ACU would have a dramatic improvement in this type of performance.
> 
> Even just replacing the A4 and A5 cards with even a simple CPU.  The
> existing system is a PI scheme that will always overshoot the desired
> positon at full slew:
> http://www.aoc.nrao.edu/~bbroilo/acu_frm/specs.pdf
> 
> Adding a little D (Derivative) would improve things, but would be
> tricky to do on the analog loop computer (in fact it looks like this
> was the intent but was abandoned, probably due to time constraints or
> oscillations).  A slow (like 100Hz) programmable loop could fix the
> overshoot and reduce the time to source, and then we could implement
> better control algorithms as they are developed and tested.
> 
> However, it seems silly to do this without replacing the other guts of
> the ACU at the same time.  A PLC could do most of the positioning and
> error logic, and a small cheap processor could do the rest.  Now that
> the panic of ELVA is calming down, perhaps we should continue to
> attack this?
> 
> Bob.
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