On Wed, 5 May 1999, Phil Pappas wrote: > Hello dialists: > > I conducted over thirty different experiments using all sorts of hole, > crosshair and bead diameters. The objective, of course, was to find the > style which cast the smallest point of light or shadow onto the analemma. > > The design which worked the best was a 1/8 inch spherical bead, suspended by > thin brass crosswires, in the exact center of a 1/4 inch round hole. (The > style was about 24 inches from the analemma). > > A very curious thing happens with this type of style. The bead alone, by > itself, casts a shadow that was twice as big as the bead; but when the 1/8th > in. bead is in the center of a 1/4" hole, with a space of 1/16th of an inch > between the bead's edge and the hole edge, the bead's shadow miraculously > sharpens into a tight, dark shadow that is only 1/16th of an inch in > diameter, smaller than the bead itself!!!! The wires which keep the bead > suspended in the middle of the hole are so thin that they don't cast a > visible shadow onto the analemma.
At first glance, this sounds like you're creating a sort of zone plate lens, relying upon diffraction, as you suggest. I'll have to play with this a bit - I can see lots of applications! Since the alidade is always rotated so the light path is "on axis", the bead is always centered in a circular aperture. That being the case, a flat disk should work as well. I'm thinking of printing an image if the bead-in-ring style onto transparency film with a laser printer. That way, we can try many combinations without the effort of machining a finished product each time. For a final design, the brass parts have much more class, and never get dirty or scratched! Dave
