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Kevin R
sage
Reged: 03/29/08
Posts: 305
Loc: Fl.
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From Tele Vue's web site:
Quote:
LOW-TO-MEDIUM POWER VIEWING
For low-power viewing of large objects, or to use your telescope as a low-power finder, you will want an eyepiece that delivers close to the maximum possible true field of view (note that for 1.25" eyepieces, the maximum field stop diameter is 27mm; for 2" eyepieces, it's 46mm). Then add eyepieces covering uniform increments in smaller field stops. For example, if your widest field eyepiece has a 40mm diameter field stop and you choose a decreasing increment diameter factor of 2 (which results in a 4x decrease in area size), you'll end up with eyepieces having field stop diameters of approximately 40mm, 20mm and 10mm. To further fill in with incremental steps, add eyepieces with approximate field stop diameters of 28mm and 14mm. Of course, avoid duplicating focal lengths. For example, if you use a 31mm Nagler (with a 42mm field stop diameter), you would not need a 32mm Plossl (with a 27mm field stop diameter).
I've been driving all day so I'm probably a bit punchy right now.  It looks like they're saying that a 1.25" eyepiece cannot be over 27mm. However, I know I see 30mm and 40mm eyepieces all over for sale in a 1.25" size. Or am I missing something here...
Kevin (2)
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If you want to see the most beautiful girl in the world, click here!
Lakeland, Fl
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MikeRatcliff
Pooh-Bah
Reged: 06/12/04
Posts: 1105
Loc: Redlands, CA
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The 27mm is the "field stop", not the usual focal length number. Two different kinds of numbers.
The field stop being the effective opening diameter of the eyepiece. The wider the field stop, the wider the true field of view, regardless of magnification. True field being the piece of the sky that is being observed.
The eyepiece focal length, along with the overall telescope focal length, determines the magnification.
The 32mm and 40 mm eyepieces in the 1.25 size have relatively narrow-looking apparent fields of views, due to the limiting effect of the 27mm field stop diameter. So with these you get lower magnification and a narrower apparent view. On the other hand, an eyepiece like a 24mm Panoptic has a wider apparent field of view, a higher magnification, and the same true field of view as the 32mm plossl. Both the 24Pan and the 32plossl show the same part of the sky (true field), but the Pan has the wider apparent field (how it looks in the eyepiece).
The analogy I like to use is an older television screen and a newer wide screen TV, both at the same distance. Both show the same view of the show, but the wide screen has more "magnification" and is easier to see details.
Clear as mud?
Mike
Edited by MikeRatcliff (08/05/08 08:30 PM)
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Alleline
professor emeritus
Reged: 02/07/06
Posts: 578
Loc: Farmington, Maine
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I think that in your tired state you read "field stop," and thought "focal length."
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Tom
Meade DS-16 (F4.5), C-8, Synta 120mm F8.3 refractor.
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