PSN-L Email List Message

Subject: Re: Not so simple photoelectrics, or are they?
From: "David A. Latsch" blottobear@..........
Date: Tue, 29 May 2001 18:17:32 -0600

Hello All,

Much easier to use the linear CCD array-all
the Rube Goldberg fixes are not needed!

twleiper@........ wrote:

> 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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Larry Cochrane <cochrane@..............>