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Doug Reilly
super member
Reged: 07/29/08
Posts: 158
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Optics thought question (with a likely easy answer in the negative).
Could you take an off-axis parabaloid segment (say at f/10) and use a flat-mirror in cassegrain focus orientation? The flat would have to be large but could be entirely out of the initial light path.
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I keep a blog on astronomy, astronomy outreach, and other related topics at punkastronomy.com Nerd is punk!
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1990
Loc: Hockessin, De
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The problem is that the length of the light path at f/10 that is outside the diameter of the off-axis segment is not very long. So the light reflected off the flat mirror would not be long enough to reach the back of the primary.
If you used a convex secondary you can increase the back focal length. That is basicly what a Schiefspeigler is but they don't require an off axis segment.
You can also make CHIEFspeigler that uses on axis parabolic mirror that is tilted, a flat and two correcting lens. The light to comes to focus at point lower then Newtonian but not behind the primary as in a cassegrain.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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gregj888
sage
Reged: 03/26/06
Posts: 303
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Doug,
You could make a folded Newtonian with the flat mirror. If you want the focus behind the primary then the secondary needs to be a bit over 50% of the diameter of the primary. More details if you want them, but not to workable.
You could use a small secondary and a relay lens.
You can also make an off axis Cass or Gregorian with a convex or concave secondary. With an f/10 primary the Cass/Greg will likely be in the f/30 plus range so a spherical secondary might be good enough. Otherwise the secondary needs to be off axis or at least mounted that way.
Greg
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Doug Reilly
super member
Reged: 07/29/08
Posts: 158
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David and Greg,
As I figured, both reasonable reasons why it wouldn't work. Thanks for humoring me. Cheers
doug
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I keep a blog on astronomy, astronomy outreach, and other related topics at punkastronomy.com Nerd is punk!
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 669
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DOUG look at the files on spiderless yahoo group
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