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Naysmyth Focus

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Posted by Robert May on June 8, 2005 03:25:07 UTC

The Naysmyth focus is nothing more than taking the focal point of a Cassegrain (other designs qualify also but it is not as useful as the Cass designs) telescope out one of the Declination bearings rather than piercing the primary.
When you get to large deep primaries, you have to calculate the depth of the curve in the glass to start with. 30" at F4.5 (135") means that the formula:
S = R - sqrt(R^2 - r^2)
S = 270 - sqrt(72900 - 225)
S = .416" = 10.6mm
This is about 1/3 of the thickness of the glass so you don't have any wiggle room when it comes to the thickness in the center. I'd sort of prefer to get the glass slumped to the ROC first and then you just have to do fine grinding and have that full thickness through the entire mirror.
There are some ATMers down under but I don't know where they have been getting their mirrors coated. There are also several large astronomy organizations around the land and they'd probably know.

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