FirstSight
Postmaster
Reged: 12/26/05
Posts: 7676
Loc: Raleigh, NC
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All Naglers of whatever type, focal length, and barrel size have an 82 degree AFOV, which I understand means that the visual angle the eye sees in any direction from the central or "zenith point" in the field of view out to where the field stop edge is visible is 82 degrees. Yet, the view appears significantly wider and more immersive in a 26mm Nagler T5 than say, an 11mm T6 (or any other T6). The main physical difference in the (outermost) eye-facing lenses of the respective EPS is that the visible lens diameter of the 26mm T5 is apx 24mm, vs. apx. 18mm for the 11mm T6. To my eyes, this ratio (24/18 or 4/3) corresponds roughly to the difference in what I'll call "apparent immersive angles of view" - a quite noticeable perceptual difference, despite the two EPs having the same nominal AFOVs.
Mind you, this is not in any sense a complaint against the T6 Naglers - to get any of mine away from me, you'll have to pry them from my cold, dead fingers.
Rather, it's provoked my curiosity about the "apparent field of view" not being the whole story in producing the effective "angle of immersiveness" in the eye's perception. Part of it too, likely has to do with the wider field stop and TFOV of the 26mm T5 usually producing more stellar objects spread across (and more densely within) the viewfield than the 11mm T6 - simply because it covers a wider true angle of sky - which naturally tends to perceptually lend a further-removed, more panoramic context to the mind's interpretation of the scene. [I also suspect the lingering tendency of Naglers toward narrow "blackout" zones at various distances from the eyepiece is a price for optically achieving greater immersiveness at other eye-distances).
I realize a somewhat related thread on "why are naglers so immersive" covered some of the same ground a couple of months ago, and will re-read that for additional insight.
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Chris M., aka "First Sight"
Orion XT12i Dob with Moonlite CR-2 focuser
TeleVue NP-101 refractor
WO Megrez 90 refractor
UniStar Light mount
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square_peg
Postmaster
Reged: 03/26/04
Posts: 36712
Loc: Maple Valley, WA
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It may have to do with eye relief, 12mm for the 11T6, 16mm for the 26T5. If you're uncomfortable getting that close to the 11mm then you're not getting the entire AFOV.
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Tom (Pegster)
DSH-8 (GSO Dob)
15x70 Oberwerks
ED80/SVP
WO 66P
Sears Discoverer EQ 60/900
8x42 Regals
History is Philosophy teaching by examples.
Thucydides
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hoof
Carpal Tunnel
Reged: 04/07/05
Posts: 1523
Loc: Redmond, WA
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Also, the 82deg isn't always easy to translate into TFoV. Take for example the Meade 30mm UWA and the TeleVue 31mm Nagler T5. In star drift testing on an F/5 12.5" dobsonian, the two have virtually identical TFoV (about a second difference in a 5-minute star-drift test). This means that either Meade or Televue is incorrect on either their focal length or their AFoV, or one has a different distortion pattern to the other resulting in the Meade showing a similar chunk of sky despite having 1mm less focal length. In this case, the Meade has much less pincushion, so I suspect the distortion issue is the root cause of the difference.
The Meade 14mm 4000 series also has an oddity with its view. Supposedly it has an AFoV of 84 degrees. However, star drift testing yields a TFoV more like a 13mm eyepiece with 82deg AFoV, or a 14mm with a 78deg AFoV. Either the AFoV or FL figures are wrong, or there's a distortion factor inherant in the eyepiece. Interestingly, the Meade 4000 series UWAs have some of the least amounts of pincushion distortion I've seen in a UWA-class eyepiece (telephone poles stay straight in this eyepiece).
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Jonathan Hoof
15" F/4.14 Discovery Truss
8" F/5.9 Orion XT8i
6" F/6 Intes-Micro MN66
127mm F/7.5 Meade APO
80mm F/7.5 Orion 80ED
18x50 IS Canon binoculars
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Tom Trusock
Reged: 02/26/02
Posts: 33846
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I find large AFOV's are a necessity for immersion, but it also has a lot to do with field lens size and the physical presentation the eyepiece gives to the eye.
Tom T.
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You do not need a parachute to skydive, you only need a parachute to skydive twice...
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leukotomy
sage
Reged: 06/14/06
Posts: 426
Loc: (after extreme makeover)
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i'm having trouble with the whole notion of immersiveness since it's some sort of physiological experience that seems to defy quantification or apt description.
if a large patch of sky is like a detailed mural then there would be an ideal viewing distance for that mural where a combination of magnification and viewing angle are optimized. standing too far away from the mural is not immersive. standing too close is not immersive. i think eyepieces are not that different. i like looking at the moon with 10mm plossls in a binoviewer. that's immersive to me (engrossing) for a mere 50o afov. so it must also have something also to do with the detail of the object and not just the fieldstop, eyelens size, eye relief, etc.
of course i kind of confused matters up by mentioning binoviewer.
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Book burning is the practice of ceremoniously destroying by fire one or more copies of a book or other written material.
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