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Fabricating An Apo Refractor Objective

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Posted by Eric Nicely/">Eric Nicely on February 9, 1999 22:11:25 UTC

I'm in the midst of making an three-element apo objective which uses FK02 as the central element. The internal radii match. While I am confident about hitting the design radii within tolerance, and wedge is not a problem, getting the concave and convex surfaces to match has been a problem.

I wonder if anyone can assist me in determining how many fringes I can tolerate. I am using the first and third elements as the concave test plates to test the convex FK02 surfaces. The diameter of the lens is 120mm, R2/R3 are +/-298mm.

The closest I've been able to come is 20 fringes. Further polishing has tended to lead to the fringe pattern expanding outward from the center and inward from the edge simultaneously. I think this suggests contact at the edge and the center, meaning a depressed ring halfway across the radius of the lens. Any suggestions on working with FK02 would be appreciated.

I have made several mirrors and a Schmidt camera, but this triplet is a real challenge.

Thanks for any help!

Eric Nicely

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