Ben Hoffmann wrote: > > Dear Ross, > > I have a comment on your comment! I agree with the issues as discussed > about parallax - however, if you were to use a solar simulator like that > discussed by Thibaud Taudin-Chabot, and place the very tip of the > gnomen/style at the center of the spherical coordinate system that he > describes - wouldn't this eliminate the parallax concerns from the > perspective of simulating the shadow? > > Thanks, > - Ben
Yes. the key is the large parabolic reflector. A point source placed at its focal point, produces very well collimated rays emanating from its aperture. Since all rays are parallel, there is no parallax relative to the sun (at its very large distance away). By putting a finite source of correct diameter at the focal plane, the rays emanating from the parabola's aperture can be made to diverge to fill an angular diameter of 0.5 degree, which is small enough that the off-axis aberrations of the "paraboloid of revolution," as it is called, are minor. A problem is any obscuration of the central emerging rays which might be produced due to shadowing of the reflected rays by the light source and its supporting struts or other mechanism. This result is a hole in the illumination of the test area. I avoid this hole in the illumination in my computerized ray tracing work by making the source completely transparent, so the reflected rays go through it as if it were not there. In practical applications this problem is avoided by using what is called an "off-axis" parabola (paraboloid). The mirror is made to be an off-axis section of a paraboloid of revolution, so that the source is off to the side, away from the collimated rays reflected from the mirror. Off-axis parabolas are very expensive to construct with good quality, and the cost goes up exponentially with aperture area, so to make one capable of covering a decent sized sundial or gnomon can be a very expensive proposition. This is where war-surplus searchlights come it. They can be very inexpensive compared with original custom fabrication, especially considering their large aperture areas. Taudin Chabot's message didn't say anything about the central obscuration issue, so I don't know how that was handled by the Technical University of Delft. With such a large primary mirror, it is possible that they use only an off-axis portion of it for illuminating the test area. The radial arm has to be fairly massive with such a mirror, to support the weight of the mirror and light source. If you ever find a surplus searchlight whose mirror still has a decent reflective coating (probably aluminum with a thin silicon monoxide overcoat to prevent oxidation of the aluminum), get your hands on it and give me a call. Ross McCluney
