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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 660
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Tim the distance from the last lens to focus can be adjusted by moving the primary up and down.. if you have enough adjustment turn all three bolts one turn and see if it is better. i can move my flat to do this adjustment on the 20 inch//kevin
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Tim,
Actually the tolerance on this isn't as bad as I thought. When I increased the BFL +.5 inch from nominal the Strehl drops by .060 and when I decreased the BFL by -.5 the Strehl dropped .017. Better to be short than long but both cases are diffraction limited. So then if your mirror radius actually measures 128 inches then you could use a tape measure to measure the top front edge of the mirror to the bottom rear edge of the first lens which should read 51 & 13/16 inches. If your radius is off a little from this just scale this number proportionally.
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Ed Jones
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timo4352
super member
Reged: 04/16/07
Posts: 160
Loc: Northeast Ohio
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Thanks Ed 
And once the moon comes back - then I can try the scotch tape to focus trick too.
Tim
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Orion 3.6CA Reflector
and 2 homebrew scopes --
8" Hubble Bubble ballscope
8" F/8 CHief - nearing completion 
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timo4352
super member
Reged: 04/16/07
Posts: 160
Loc: Northeast Ohio
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Quote:
Tim,
Actually the tolerance on this isn't as bad as I thought. When I increased the BFL +.5 inch from nominal the Strehl drops by .060 and when I decreased the BFL by -.5 the Strehl dropped .017. Better to be short than long but both cases are diffraction limited.
Ed
So if I'm reading it right I really have at least one inch of room to adjust and I still wouldn't notice any difference at the eyepiece? That just made building this scope a lot easier than we thought then. Do you think?
Tim
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Orion 3.6CA Reflector
and 2 homebrew scopes --
8" Hubble Bubble ballscope
8" F/8 CHief - nearing completion
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Yes it's a very tolerant design if it can be off +/- .5 inches. It's always best to get it as good as you can though.
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Ed Jones
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timo4352
super member
Reged: 04/16/07
Posts: 160
Loc: Northeast Ohio
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Me, being the perfectionist that I am, will have to get it just right to be happy. It's good to know there's some breathing room though.
BTW thanks for putting my scope up on the spiderless homepage. It made me feel real good that you thought enough of my scope to do so.
Tim
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Orion 3.6CA Reflector
and 2 homebrew scopes --
8" Hubble Bubble ballscope
8" F/8 CHief - nearing completion
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