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Hubble Optics produce a LED torch with five precise holes, from 50 to 250 micron diameter. Around us$12...
I removed the steel plate with the holes and put in on my trusty ol' foucault tester. Now I can use a neon, fluoro or LED light at any artificial star size for testing.
Well worth the money!!
Suiter " Star testing astronomical telescopes" has all the mathematics for the size and distance of ball bearings; everything from Xmas decorations to 6mm ball bearings.
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