[time-nuts] Re Danjon Astrolabe

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Sep 28 18:37:19 EDT 2006

David Forbes wrote:
> 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.
> Bill,
> 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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The sun is about 0.5 degrees in diameter so a quad cell photodetector 
setting precision is limited by this to around 1/100 of the solar diameter.
In other words the time derived from the sun position will be accurate 
to about 1 second.
The Zeiss suntracker used to monitor 4 sectors of the solar limb to 
achieve arcsecond tracking.
They used an occulting mask at the focal plane of a tracking telescope a 
mechanical chopper and a photomultiplier followed by a phase sensitive 
detector to derive error signals for the tracking system.


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