I have no calculations. About ten years ago I erected a very
efficient solar collector to pre-heat my well water. Among
other things, it tracked the Sun in order to maximize the
yield. I built a photo-cell / optical system to do so with
much experimentation, but ran into the challenge of the
diffusion caused by overcast days, non-linear cell response
and so forth. So then I added a "jog" cycle that would just
reset the collector to due east every morning and then drive
it westward for a few seconds every so-many minutes, and
that would keep the dual photo-cell detector close to target.
This worked fairly well until a thunderstorm took out the
photo-cell amplifier that was located up on the collector,
at which point I realized that the jog-cycle method alone
was equally good and far less trouble. That's the last
time I used photo-cells to detect anything.

The method I speculated earlier today was off the top of
my head. But it does make sense to see if the numbers
agree with my instinct. A modest attempt follows:

It seems reasonable that the "angular amplifier" I
described using two opposed mirrors that move slightly
off parallel should work and, if you were spinning the slit
at around 30 revs per second any noise and instability would
be effectively averaged out for any but the shortest period
instruments. And, depending upon how many times you
bounce the path between the mirrors it seems reasonable
that you should get a amplification of angular displacement
of the boom by at least an order of magnitude, meaning that
the .01 arc/sec requirement becomes .1 arc/sec. This could
also be expressed as about 1/200,000 of a revolution and
would occur in about 160 nanoseconds in the 30 RPS example.
Detecting phase shifts in 160 ns chunks is a piece of cake,
in fact you could probably go down another order of magnitude.

All I know is that I can track a radial on a VOR to the degree, and
that thing also uses an antenna that's spinning at 1800 RPM ...
AND you have all sorts of variables like RF propagation. It seems
that with light, good design and controlled conditions (all of which
you can provide) one should be able to do at least a thousand
times better.

On the other hand, I have a cat that always seems to sleeping
on the sunroom sofa whenever a major quake occurs around
the world. Maybe I can put that to good use...

Tom

On Tue, 29 May 2001 17:04:18 EDT ChrisAtUpw@....... writes:
> In a message dated 29/05/01, twleiper@........ writes:
> 
> > All this talk about precision matching and ambient light,
> > spectral response, etc., seems crazy. If you really must use
> > a photographic system (without film on a drum) why take
> > 
> 
> Dear Tom,
> 
>        Was anyone talking about photographic techniques? We were 
> considering 
> the precautions / choices necessary to get optical detection methods 
> to work 
> OK for very small movements. How would your method cope with 
> movements from 1 
> sec of arc down to maybe 1/100 sec of arc? I would be very 
> interested to read 
> your calculations.
> 
>        Regards,
> 
>        Chris Chapman
> 
> 
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