Hi Rich: --- Richard Tenney <retenney@yahoo.com> wrote:
The kits sound like a terrific deal. BTW, I start a new job on Tuesday! Yahoo!
Hey, great news! Congratulations!
In other words, is it possible to make a 6mm ortho that fits in a 2-inch barrel that doesn't require the top lens element diameter to be smaller than a soda straw?
No. Lets assume that an eyepiece is just one fat lens for the purpose of argument (which is the way the eyepiece works in practice, anyway). Think f-ratio of the eyepiece lenses, not just focal length. If the eye lens were say, 20mm in diameter, the FOCAL RATIO of that lens would have to be about F/0.3 to achieve the desired FOCAL LENGTH. Lenses of F/0.3 do not form very good images! The "sweet spot" of diffraction-limited performance would be microscopic, if there was one at all, and the rest of the field would be horribly curved. Just like our telescope objectives, longer F-ratios mean better imagery. So, to get a larger distortion-free zone, we have to change that F-RATIO. To keep the focal length the same, you have to reduce diameter to increase the RATIO (like a camera lens iris). Get it? Short focal-length eyepieces with large eye lenses are in fact augmented with internal Barlows. In this case we are cheating by actually increasing the apparent focal length of the objective...remove the internal Barlow, and those eyepieces are really more like 20-25mm eyepieces. UO some years ago had a high-powered eyepiece line in 2" barrels, but they didn't sell. The larger barrel added to cost & weight while adding nothing to performance. Remember that with high-powered eyepieces, you are only looking at a tiny portion of the virtual image at prime focus....you don't need a 2" barrel. Most of a 1.25" barrel is wasted, for that matter. Long eye-relief and wide field can be mutually exclusive qualities, I'll demonstate this graphically at the seminar. Imagine looking through a 1" knot-hole in a wood fence. If your eye is right up against that hole, you can see quite a panorama on the other side....Now, back your head back from the hole a bit, and you can see how that panorama shrinks...a similar effect is taking place with eyepieces. In practice, a wide field can be obtained by making the "field lens" of larger diameter than the "eye lens" in any given eyepiece design, not too hard to do. Now you're looking through a funnel from the small end, instead of a tube of constant diameter. This is all pretty simplified, but we'll get a little meatier at the seminar.
One of the things that Mr. Dobson said about binocular eyepieces were the superb eye relief most of them afforded.
Some, yes, but the cheapies usually use 3-element Kellner variants with marginal eye-relief, but wide field. Remember the eye-relief/wide field tradeoff. I know the eyepieces on the Obies/Bears/Galileo 15x70mm binos are a great compromise...wide field and reasonable eye-relief.
Anyway, sounds like you might need to start the seminar with a short course on optical physics! I for one sure wouldn't mind.
Meanwhile, I'll have to go back and look up your S&T article. I never did read it that I can remember.
Shame! Shame! You're off my Christmas list! ;) I didn't write those for the money, you know! (well, OK, I did a little bit....) Chuck __________________________________________________ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com