Re: Shadow caster

[Tom Egan]

Dave, Speaking of a bright sun spot, consider the following setup: A hemispherical convex mirror mounted above a (hopefully clear) ceiling window so that you can see, from inside the room, the bright reflection of the sun A dial face, horizontal, on a table within view of the mirror A pinhole in a mask of the right size (will explain below), horizontally suspended somewhere on a line between the mirror and the dial face (placement explained below) I've made this sort of thing work at a distance roughly half of yours using ... A 26 inch diameter ellipsoidal weatherproof mirror A 1/8 inch diameter pinhole A letter-size dial face. If d = diameter of mirror, w = longest dimension of the dial face, a= distance above pinhole to the center of the mirror, b = distance below pinhole to the dial face, L = a +b, and m = width of the mask (large enough to keep the dial face in shadow from extraneous light from the mirror), then b = L[w/(d + w)] m = w + (b/L)(d-w) Assuming for your application L = 10 ft, d = 26 inches, w = 13 inches, we get b = 10/3 ft and m = 17 1/3 inches. In other words, if you hang a 17 1/3 inch diameter mask, with pinhole, 3.33 ft above the dial face, the mirror will reflect a sun spot onto your 13 inch dial face that should not stray off the face no matter the time of day or the day of year. (This assumes you place the dial face approximately under the apex of the south-facing hemisphere that has a clear view of the sun throughout the day and year; if you place it much off from that, the math would have to be elaborated to take the angle into account.) The spot is bright enough to see in ambient light in my application.  Further, a discernible spot will be formed as long as the sun is bright enough to cast a shadow. Tom On 3/4/2012 12:33 PM, Dave Bell wrote: I agree that a bright sun spot would be nice, but I don’t want to black out much of the window.   Unfortunately, over the years, the double-glazed window has grown a spotty diffusing layer of something (mold? dried rainwater seepage?) that makes it less than ideally clear.  I tried a solid, 2 inch spot of aluminum foil, and the shadow is barely visibly 8 feet from the glass. I don’t think the glazing is something I can disassemble for internal cleaning…   I’ll print some positive and negative zone plates in different sizes this week, and see how they work.   Dave   From: Reinhold Kriegler [mailto:reinhold.krieg...@gmx.de] Sent: Sunday, March 04, 2012 11:51 AM To: 'Dave Bell' Subject: AW: Shadow caster   ☺ I thought so!!!   Actually: Inside the house a little sunspot is much more fun than a tiny dizzy and diffuse shadow point…   Reinhold   * ** *** **** ***** ****** ******* Reinhold R. Kriegler Lat. 53° 6' 52,6" Nord; Long. 8° 53' 52,3 Ost; 48 m ü. N.N.  GMT +1 (DST +2)   www.ta-dip.de http://de.youtube.com/watch?v=XyCoJHwzzjU&fmt=18 http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html http://www.ta-dip.de/dies-und-das.html -----Ursprüngliche Nachricht----- Von: Dave Bell [mailto:db...@thebells.net] Gesendet: Sonntag, 4. März 2012 20:35 An: 'Reinhold Kriegler' Betreff: RE: Shadow caster   Yes, I remember John’s work well, particularly for gnomon points, so the shadow isn’t truncated.   I was thinking about putting a shadow spot on one of my living room roof windows, particularly with the approaching Equinox. But over an 8 to 15 foot throw, the shadow would be fairly diffuse.   I’m going to try printing some large (1 to 3 inch outside diameter) zone plates on transparency film, and see how they work…   From: Reinhold Kriegler [mailto:reinhold.krieg...@gmx.de] Sent: Sunday, March 04, 2012 10:05 AM To: 'David Bell'; sundial@uni-koeln.de Subject: AW: Shadow caster   John Carmichael has done excellent research work about this question! Just contact him!   Regards! Reinhold Kriegler   * ** *** **** ***** ****** *******   Reinhold R. Kriegler   Lat. 53° 6' 52,6" Nord; Long. 8° 53' 52,3 Ost; 48 m ü. N.N.  GMT +1 (DST +2)   www.ta-dip.de   http://de.youtube.com/watch?v=XyCoJHwzzjU&fmt=18   http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html http://www.ta-dip.de/dies-und-das.html   -----Ursprüngliche Nachricht----- Von: sundial-boun...@uni-koeln.de [mailto:sundial-boun...@uni-koeln.de] Im Auftrag von David Bell Gesendet: Sonntag, 4. März 2012 18:50 An: sundial@uni-koeln.de Betreff: Shadow caster   Several times, there have been discussions about how to improve the shadow cast by a "point" nodus. I partially recall some conclusions regarding the optimum diameter vs, throw length, and some thoughts about adding an annular ring to take advantage of diffraction.   Can anyone help remind me? Thanks!   Dave   Sent from my iPhone --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial

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