[time-nuts] Re Danjon Astrolabe

David Forbes dforbes at dakotacom.net
Thu Sep 28 17:09:10 EDT 2006

Bill Hawkins wrote:
> Tom Van Baak wrote,
> "2) Instead of a fixed base, gnomon, and slowly moving shadow like
> almost all sundials, you put a stepper or servo motor/encoder on the
> base. Then place matched photodiodes on either side of the gnomon and
> steer the whole sundial for constant *minimum* shadow. In real-time, a
> 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.


Back in the good old days before CCD arrays, people in the astronomy 
business used quadrant detectors for this sort of gizmo. A quadrant 
detector is a 2x2 silicon photodiode array. When the bright spot is in 
the middle, then the current through all four diodes is equal. When the 
object is off-center, the current is unbalanced. You can make a tracking 
servo using this detector that's entirely analog - no programming skills 
required! Of course, driving the alt-az mount requires derotating the 
detector array relative to the mount's alt-az axes.

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