Originally published at: http://boingboing.net/2017/06/16/heres-what-the-world-would.html
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The air currents were pretty cool but then the explanation of his set up was even more more awesome. I learned new things today and it’s only 7:30. Good start!
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Isn’t that what everything already looks like… wait, um?
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Air currents? How primitive. Your species could measure those centuries ago. WE shall be impressed when you learn to image the weak nuclear force.
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I used this when polishing an 8" telescope mirror. It is a wonderful technique that shows the errors from a spherical surface as though they were mountains lit from the side away from the razor blade. I am guessing the ray-like pattern that does not move is an error in his mirror figure. I could see errors of an eighth of a wavelength of light, and the expansion in the glass where I had handled the mirror a quarter of an hour ago.
I had a quartz-halogen lamp in a tin with a pinhole, as there were no LEDs then. I had an unsilvered mirror. I stuck the lamp where he put the LED, and put my eye where he put the camera…
…and felt a total idiot when I cut my nose on the razor blade.
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Was wondering how many animals could make that statement, and what it would look like if we perceived the world as they do.
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You’ve made an interferometer! A telescope uses a parabolic mirror, as you described, but for an interferometer you’d generally want a sphere. Since for an f/7 (f/8? I’m guessing here) parabolic mirror you’d only want about a quarter fringe (1/8 wave of light at visible wavelenghths) of deviation it doesn’t show too much deviation at the center and edge. For a long focal length telescope mirror (f/10, say) you don’t need any parabolic correction and a sphere works fine (your more magnified object will fly out of the eyepiece quicker on your extra long scope, that’s all). It’s fast mirrors like f/6 and below where a 'scope mirror won’t work in this set-up.
This would also be useful for checking the figure progress of a home-made telescope mirror.
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I love the Veritasium channel. Hours and hours of cool videos to watch. Almost always some new tidbit to learn.
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It’s not really interferometry. There is Zernike and Tolansky interferometry, which need a reference beam so are hard to do at these scales; and time-of-flight interferometry, which only gives interference when the primary and the reference have similar path lengths, so can be done at these scales with much rougher apparatus. This has no secondary beam to use as a phase reference. There are no interference fringes - just a black to white contrast shift, which is the same for all wavelengths.
You could have a colour version with two perpendicular coloured filter edges cutting the focus into four, giving the appearance of the errors from spherical shown in relief, but lit from two directions.
PS : my mirror was a f/8, and it needed parabolisation, but not much, and this was done by eye.
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Fair enough. You’re right, no reference beam so no phasing. It’s good to see this kind of modeling and thanks for keeping optics in boingboing’s vision.
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I thought I understood what he was doing until he introduced the razor blade to the setup. Huh? "And now of course we also need to duct-tape a banana or any of the Musuceae family (plantains are ideal) to the back of the mirror…"
Also, what happens if, for your “point source” of light, you used a laser pointer?
A laser pointer would only illuminate one small point on the mirror. You want a uniform illumination over the face of the mirror from a single point source.
A banana carefully trimmed to look like a razor blade would work equally well. … for a little while. It would certainly be safer, right Richard?!
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A laser pointer with the lens removed diverges around 15 degrees, so it might work. Or be too dim at that amount of divergence. Who knows. https://www.thorlabs.com/tutorials.cfm?tabID=f7ed0dd5-3f31-4f84-9843-e0f7ac33f413
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