Re:Telescope Building with John Dobson (2014) [video]
by tzs on Saturday July 08, 2017 @02:02PM
(#14726423)
The thing I find amazing about home made telescopes is how accurate you can make the curve of a hand ground mirror with such simple equipment. You have some intended curve you are trying to make, and you need to hit that to within something like 1/4 of the wavelength of visible light or better (call it 100 nm).
Most people would expect that seeing such small deviations would require expensive equipment that is complicated and has to be built to insane tolerances. Nope. All you need is a point-like light source, an opaque item with a sharp straight edge, and a dark room.
The point-like source can simply be a light in a can or box with a small hole for the light to escape. The sharp straight edge can be a razor blade.
With that simple equipment you can do a Foucault knife-edge test [1]. That lets you figure out what parts of the mirror are too high or too low from the intended curve, and you can use that to guide your further grinding of the mirror to bring it to the right shape.
Re:Telescope Building with John Dobson (2014) [video]
by dekhn on Sunday July 09, 2017 @10:45AM
(#14729945)
You should also mention that the almost perfect curve can be generated automatically by just rubbing two rough surfaces (hemisphere and sphere) together.
Most people would expect that seeing such small deviations would require expensive equipment that is complicated and has to be built to insane tolerances. Nope. All you need is a point-like light source, an opaque item with a sharp straight edge, and a dark room.
The point-like source can simply be a light in a can or box with a small hole for the light to escape. The sharp straight edge can be a razor blade.
With that simple equipment you can do a Foucault knife-edge test [1]. That lets you figure out what parts of the mirror are too high or too low from the intended curve, and you can use that to guide your further grinding of the mirror to bring it to the right shape.
[1] https://en.wikipedia.org/wiki/Foucault_knife-edge_test