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For planetary alone this makes no difference, but the point is that full (100%) illumination of that 42mm focal plane, when it's located 5" from the optical axis, occurs with a minor axis secondary of 1.42", which leaves your 8" f/8 working at a secondary obstruction of less than 18%. This system will equal or beat any 6.5" unobstructed (be it APO dripping with exotic glasses, or unobstructed mirror design, or some combination thereof) in contrast transfer of planetary detail, though it will give up some ground on off-center abberations for highly corrected designs. OTOH it will have no spurious color and the center of the field will have zero aberrations.

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Mark:

It would have been lovely to have your participation a week or so back on the "Aperture Rules" thread over in refractors. A bunch of APO fanciers claiming a 6-inch APO will outperform any reasonably sized reflector... In fact, touting their resolution advantage over 20-inch reflectors. I realize rational arguments make no difference under such circumstances. It still would have been fun.

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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com

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