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KennyJ
Reged: 04/27/03
Posts: 10029
Loc: Lancashire UK
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< Am I on the right track, or am I missing something? >
Peter , I'm not sure if YOU are missing anything , but I'm beginning to think that my eyes must be ! :-)
Having earlier today congratulated Glenn on his ability to explain his thoughts without diagrams or sketches , what do I find in his next post -- ?
-- but a link to illustrations ! :-)
Kenny
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Two eyes and a preference to use both
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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GlennLeDrew
sage
Reged: 06/18/08
Posts: 464
Loc: Ottawa, Ontario, Canada
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Quote:
Hi Glenn,
Nice illustration, thanks.
Several times you've mentioned "standard Porro prisms" as opposed to "asymetric Porro prisms". Is the use of asymetric prisms an attempt to correct the problem of "too small prisms"? I imagine that using progressively smaller prisms would reduce the overall prism path length and move the prisms further away from the objectives. Am I on the right track, or am I missing something?
Peter
I use the term "standard" for those systems that have equal size apertures at the entrance and exit of the prism train. More precisely, I mean that both prisms are identical in size (some binoculars may actually have the rear aperture stopped down a bit in an attempt to provide a baffle to block non image-forming light scattered off the prism walls.
Asymmetric prisms are indeed used so as to be able to accommodate at least the on-axis portion of the steeply converging light bundle. All that's needed is to make the front aperture, and its immediately following 45 deg. reflecting surface, a bit larger. In the illustration you can imagine how, if the front aperture was made just about 10-11% larger (with a concomitant but sufficiently small increase in total prism system length), the full light cone could make it into the prism.
My Celestron 25s100s use just such an asymmetric prism arrangement, and in spite of the ~f/3.85 objectives, the instrument works at full aperture. If they didn't do this, I'd never have bought them.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12341
Loc: Cumberland, R I , USA42N71.4W
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Quote:
Ed,
Measured the length of the 22x70's to be 15 7/8" with the eye cups rolled down. The eye lens was recessed ~3/8" and the edge of the objective was recessed 7/8" with the dew shields retracted. That leaves ~ 14 5/8" from end to end. With the eye cups and dew shields extended the total length is 18 3/4". The focal length should be ~ 16.5". I'm guessing 20mm FL (0.8")eyepieces.
Do you think the prisms path can account for the ~ 2.7" difference? Still seems pretty close to f/6.
Peter
typical prism light path can range from about 80mm for very small prisms up to about 120-130mm for very large prisms. Focal length thru the prisms is light path divided by index of refraction, so the prisms can reduce the overall length by 80/1.56=~50mm to about 130/1.56=~80mm. F/6 works out well.
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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