[time-nuts] Re Danjon Astrolabe

Tom Van Baak tvb at leapsecond.com
Thu Sep 28 19:40:56 EDT 2006

> The scheme probably needs three photocells to be sure that the one
> in the middle is darker than the others. Might be able to mask it
> with a slit and use a fine wire gnomon, in a coarse/fine servo.
> Could use a variable frequency motor and precision reduction, like
> a phonograph turntable only much slower.

Not sure about needing three. This is what I had in
mind -- given there's a stepper/servo on the sundial
base, I suggest a detection method not unlike the
way a 5061A stays on the cesium resonance peak.
(see http://www.leapsecond.com/pages/cspeak/)

Namely, you continuously sweep the base a small
amount, perhaps a fraction of a degree, at a couple of
Hz rate, back and forth across the minimum gnomon
shadow to determine just where the "zero crossing" is.
The two photodiode *differential* is all you care about.

Using a sampling ADC (free inside most uC these
days) you can then infer where the center is. When
the virtual sweep center is off by more than one full
physical step of the geared stepper, you advance
the sundial base one step.

I've not done the math, but I suspect you can get quite
accurate this way. Not only do you have the digital
step count as a function of time of day, but you also
have the digital/analog interpolation of the steps; so
the resolution is quite good.

Also if the photodiodes are sensitive beyond visible
this digital servo'ed sundial might work in cloudy
weather too -- which, being here in Seattle, is one
reason I came up with the idea.


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