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Re: Newtons Rings

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Posted by Dave Hawlins/">Dave Hawlins on April 15, 1998 15:04:17 UTC

: Now I understand what you are saying. When you press the master surface against the tested surface, the rings go to the center! This happens when the radius has gotten too large. The air gap is larger in the middle than at the edge. This means that there are more wavelengths of light between the two surfaces. When the rings go away and become less then the surfaces are being bent to a more similar radius. This is typical for grinding because the center tends to hold unbroken grains better. If you are only 2 half (each ring is a half wavelength) waves from what you want, you are very close to what you want. One thing that I don't understand is what you are striking the convex surface against. Is it the polishing surface that you are using or is it another surface that you have already made for a reference? If it is the former, then you have absolutly no idea of where your actual curve is unless you do a full test on it before using it as a reference. I would do a startest at this time and see if the optics is good enough. Yes, a bright star will reflect enough to do a star test with. I have split the double double with a 6" at riverside that hadn't been silvered yet. It was a mirror that I helped Bob Geoff grind and polish one year at the conference. If you're going there, I will be having a 4.25" F44 scope there. Have Fun and Keep Looking Up. Bob May

Hello again Bob, Just to clear up the misunderstanding I`m testing against the glass tool the secondary was ground on,I gave it a little polish and carried out the focault test for curvature etc. Iwould like to come to the conference but it`s along way from shropshire Enland!!. take care Bob and thanks again. Dave

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