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