[time-nuts] Re Danjon Astrolabe

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Sep 28 18:46:46 EDT 2006

Dr Bruce Griffiths wrote:
> 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 modern equivalent of limb sensing would be to image the sun onto a 
CCD or equivalent image sensor and use image processing techniques to 
accurately locate its rim and thence derive the position of its centre. 
An neutral density filter/IR blocking filter over the objective may be 
necessary to avoid destroying the CCD image sensor.


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