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GlennLeDrew
professor emeritus
Reged: 06/18/08
Posts: 633
Loc: Ottawa, Ontario, Canada
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Re: New Garrett RA 70mm's
07/22/08 08:19 PM
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In the 88mm aperture version, which I'm pretty certain use identical prism assemblies, the relatively smallish rear prism aperture imposes a limit on the widest true field. Moreover, the focal position is only a quite small distance behind the rear prism aperture, which exacerbates this. And another consequence is that dust can be seen almost in focus during daytime observation.
For the 88's, the 20X eyepiece set has field stops as large as can be used before prism aperture-induced vignetting rears its ugly head. (In fact, the 32X oculars, with their wider AFoV, have true fields not too much smaller than the 20X jobs.)
You might find that the 16X oculars in the 70's, like the 20X for the 88's (with an AFoV of ~52 deg.), offer the lowest usable power if ridiculously small AFoV's are to be avoided.
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It is possible to use 1.25" telescope eyepieces with this series of RA bino if:
1) the field stop lies at or quite close to the bottom of the barrel,
2) the barrel is long enough place the field stop just about up against the rear prism aperture, and
3) you make a "shim" to widen the barrel. Even tape will do.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
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