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. >> >> >> >> _______________________________________________ >> time-nuts mailing list >> [email protected] >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >>
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. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
