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GlennLeDrew
professor emeritus
Reged: 06/18/08
Posts: 620
Loc: Ottawa, Ontario, Canada
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Re: Oberwerk 22x100 vs Celestron 25x100 Minireview
06/19/08 02:51 AM
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A friend's "Antares" brand 22x100, which looks exactly like the Oberwerk, has what appears to be incorrectly placed and/or wrong-sized prism assemblies. So much so that they are only "collimated" by having *both* prism assemblies wickedly tilted in a complementary manner (i.e., it's no accident). At any rate, the entrance apertures of the first prisms are clearly displaced laterally off-axis. The result: a tilted focal surface, with the concomitant systematic focus shift in one plane! Not to mention the slight elliptical "cat's eye" exit pupils.
His solution: make his own Frankenstein, using the objectives (which are of good quality.)
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I have the Celestron 25x100s. Love 'em, especially after my modification (to be outlined in a separate topic.) Edz's determination of an effective aperture of 90mm doesn't square with my experience. I get virtually 100% objective usage.
Now, in making my modification I've had them open. Guess what I found. The first prism is of the "tapered" type. More accurately, it has a stepped width, with the side intercepting the incoming light cone being significantly wider. This is why, even with the "fast" objective, practically all on-axis light is being fed to the eyepiece.
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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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