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RE: RE: Proper Focal Length

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Posted by Long focal length (plenary) Newtonians. on September 15, 2000 20:51:20 UTC

As someone who also likes looking and Jupiter and Saturn a lot, I find a 4.25" f/12 (54") with a TINY 0.7 secondary great, especially with the low diffraction 3-vane spider and 1/20 wave quartz secondary (from Protostar). This scope is an attempt to reproduce the contrast and planetary quality of of a 4" APO refractor without the $2000 price tag.

It`s easier to figure a long focal length mirror. A 4.25" f/12 is an tenth wave-plus when left spherical, and the three vane spider and tiny secondary add virtually no diffraction/contrast problems. No coma either!

If you want you scope to be "planetary-capabile," and don`t mind a little bulk, figure your mirror to 8" f/7 as best as possible, get a three vane spide from pronto, along with their 20th wave fused secondary, on the smaller side, with a low profile focuser. Jupiter actually has a very busy atmosphere. Also good for getting detail out of subtle nebula and splitting doubles.

I say as long the eyepiece doesn`t rise above the standing eye height of the observer, then you can make the scope that long with better optics/

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