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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1533
Loc: Prescott, AZ
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Quote:
The BT 80's look a bit more easy to handle actually, same setup but less weight, less aperture as well but wider field... argh so many possibilities.
Even though they are "giant" binoculars i'd like them to be quick and easy in setup.
Those BT's shure seem the most interesting, price is ok, specs are ok on paper, but reviews are ok as well, looks easy in handling, looks pretty slick in fit and finish.
Yes, the BT series has been on my back-burner wish list for awhile. I keep holding out for 90 degree versions, we'll see what spring brings. But in my comparisons, I felt that the BT80 would not be the way to go. The reason being the focal length is 520 mm, which IMHO is not going to yield a vastly larger true field - 3.0 degrees vs. 2.5 degrees for the BT100. Two and a half degrees still frames most objects well. Yes, there are a few that come to mind that require more (M31, the Veil), but three degrees is not really sufficient there either.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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KerryR
sage
Reged: 12/05/07
Posts: 324
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Yeah, the 43deg AFOV of the 40mm Plossl is kind of narrow. But, I'm not aware of an eyepiece that will get you above the 2.9deg TFOV anyway (in the BT100-45). And, for me, the Plossl was the only affordable solution for the widest field and largest usable exit pupil. You can get the same filed of view with a higher power, but the exit pupil gets smaller. I went for the 7mm exit pupil (40mm eyepiece) to try to get the brightest image possible, under the assumption my pupil could dialate to 7mm. (which it probably doesn't)
One thing I didn't mention is that the Oberwerk BT100-45's are really heavy, in case you didn't notice; 26 pounds. So, they are surely not grab and go set-ups. The binoculars can not be carried out the door mounted on the tripod, as you might with the 80mm version.
Kerry
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Kerry
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Gordon Rayner
professor emeritus
Reged: 03/24/07
Posts: 506
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There must be compromises in changing eyepieces , from different designers and manufacturers, even at f/6, which is relatively slow for prism binoculars. I experienced disappointment, and wasted machining time, trying to adapt varous widefield eyepieces to WW II mounted binoculars at f/3.5 and f/4.5. , and experienced a few brief, inconclusive handheld eyepiece changing experiments with the Fuji 25 x 150 and the Nikon 20 x 120, both f/5. The original designs must have included the eyepiece in the aberration balance. The problem is certainly nonlinear, but I do not know the inverse power of the f/ratio in which the problem might, in general, be described for rule of thumb purposes in decisions about what is acceptable.
Have some of the designs which have been discussed been made such that(with compromise away from an ideal) the objectives and prisms are balanced against each other, insofar as possible, to allow the use of eyepieces which were optimized for use in prismless reflecting telescopes or highly corrected prismless refracting telescopes?
The prewar Zeiss three eyepiece turret Starmorbi 60mm, Asembi 80mm,and Asenglar 110mm collectors items, all prismatic, all worked at around f/6 or f/7, as I recall, but none had eyepiece fields of more than about 50 degrees. One assumes that the eyepiece aberrations were balanced against those of the prisms and objectives.
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Mr. Bill
Carpal Tunnel
Reged: 02/09/05
Posts: 2759
Loc: Just passing through.....
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24 Panoptic in BT100 45....25x, 2.7 degree fov, 68 degree afov, 4mm exit pupil. The best tradeoff for f/6 1 1/4 ocular.
Work great...hits the "sweet spot" for these optics.
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10x50 Fujinon FMT-SX binos
15x70 AP binos + Paragon p-mount
Oberwerk 100BT 45 degree + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 homemade achromat....EE Barnard MW Sweeper
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery split tube
35mm Pan, 26mm Nagler, 17mm Nagler, 13mm Ethos, 8mm Ethos
Member IDA
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12592
Loc: Cumberland, R I , USA42N71.4W
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As I've recommended before, you also should read the Mini Review thread that discusses "Eyepieces for the BT100". There are many that work excellent in nthe BT100, however I'd pause before choosing 40mm eyepieces. (1) Since Tfov is dependant on the diameter of the field stop in the eyepiece, you can get the same fov with 32mm, some 30mm, some 26mm and even the 24mm Panoptic. (2) Using an eyepiec that gives maximum exit pupil to the eye will contribute the greatest abberations at the edges of the eye. (3) it will only be the brightest IF your eye pupil does indeed open as wide as the 6.7mm that it provides. If your eye pupil happens to be 6mm then you are using only 6/6.7 x 100mm = 90mm of the objective lens.If your eye happens to be 5.5mm then you could achive the same brightness by using a 32mm eyepiece. (4) Slightly higher magnification will show more stars.
For those wishing to optimize a maximum 60° Afov eyepiece, the longest 1.25" eyepiece that has a real 60° Afov is a 26mm eyepiece. The Meade 5000 26mm plossl will give you that 60°, but the eyepiece is best used in f/10 or f/12 scopes. In fast scopes it has rather mediocre performance and does not perform as well as any that I mention below.
Eyepieces that I've found that work exceptionally well in the BT100 include
32mm TV plossls
26mm TV smooth-sided plossls (best - low power favorite)
20mm TV plossls (best - mid power favorite)
18mm Tak LE
18mm Celsestron Ultima plossls
17mm Orion Sirius plossls
14mm TV Radians (best - high power favorite)
12.5mm Celestron Ultima plossls
12.5mm UO orthos
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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KerryR
sage
Reged: 12/05/07
Posts: 324
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In retrospect, I probably should have gotten 32 or 35mm EP's if I was going to go with Plossls, but, when I made the purchase, I was fixated on the 7mm exit pupil thing (unnecessarily). I'll likely get a couple of 32mm Plossls at some point-- higher power at the same FOV, etc.
I agree that the 40mm probably aren't the best choice, but they do work well, and I don't mind the 43deg AFOV because the TFOV is really what counts, and I'm not losing anything there. More mag to show more stars would be my primary interest in getting 32-35mm instead. (along with the fact that it's unlikely I dialate to 7mm)
Luckily, the edge abberations are minimal, both in the scope, and apperently in my eye, with the 40mm plossl.
I like plossls because they function quite well in faster scopes. Seems like the only other design I've seen that work as well in faster scopes (and have a wider AFOV) are Panotics, Naglers, and some Harry Siebert ep's (his modefied Orthos come to mind). Because you need 2 of every ep you want, the money can pretty easily become the determining factor, especially if you have other scopes you use/spend money on.
I tried a 21mm Harry Siebert ep that had 70deg AFOV, just on one side of the bino (only had 1) but found the edge abberations to be too much to bear. My only Nagler, a 7mm, worked well, but I didn't want to spend the money on another one, especially for a power that's a little much for the BT100. I also found the 32mm TV plossl to work very well, but it's very expensive for a plossl, and because I found fatter ep's like this one to hit the bridge of my nose, I shy away from fatter ep's, and went with a thinner plossl, at the cost of AFOV.
I'm happy with the ep's I got for this scope (all GTO plossls from Hands On- $25ish each...40,25,15,9mm). I'm sure I could do better, but I like having the variability of powers that I was able to afford in this way. The GTO's are very good ep's-- on par with any other plossl, at least to my fairly un-critical eyes.
I'd definitely default to edz post and article ref, along with the other posters who mention the 24mm Pan (if you can afford/justify 2 of them-- I'd rather have 1 13mm Ethos for my other scopes). Had I been more active on CN at the time I was making the purchase of the ep's for the BT100, I could have made slightly better choices...
Kerry
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Kerry
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Muffin Research
scholastic sledgehammer
Reged: 09/28/07
Posts: 805
Loc: Belgium
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Ok going over this a lot, making a nice list of a BT 80 setup (lighter weight) inexpensive Binocular.
That I prolly like a lot for touring the night sky, at home and especially 650 miles away under dark skies (but I only spend a week a year there)
I still got a C8, now a binoviewer with that would that not blow the views with the BT 80 or even 100 away? Ok I can't get as wide field and low power with them... yes that would be a drawback and is in direct violation of my first idea of low power wide field, true true but still ...
I could alsot use them with the Genesis bringing the widefield back.
And in the meantime I already placed an order on a Canon 12*36 IS as I don' t have bino's anymore, had an old Prins armygreen thing since I was a kid, pretty solid but it broke a few months back.
and I never used it much for astronomy anyways magnification was to high, no stand, lot's of jitter.
So yes I'm a bit torn now, the appeal of the BT's is great but maybe I could spend my money better and more versatile if I go over to the other forum with binoviewers instead of giant binoculars... ARGH
who said that choice was freedom?
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Quelle Horreur!
Requime pour un Twister.
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Joe Ogiba
Post Laureate
Reged: 02/14/02
Posts: 3362
Loc: NJ USA
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If I had the money I would love to build a binoscope using a pair of Pentax 100mm F4 APO's with 22mm Naglers and 13mm Ethos eyepieces. 
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Pentax PF-80ED
Meade 102ED APO
Orion EON 72
120ST
Apex 127
C6 XLT
CR150
C9.25
XT10
Zeiss 7x42 FL
Canon 10x42L IS WP
15x50 IS
12x36 IS II
Garrett Optical 28x110 HD-WP Signature Series
Oberwerk BT-80 45
Apogee RA-88-SA
Denk II Power x Switch binoviewer w/13mm Ethos, 20mm Pentax XW's, 20mm Widescan III's.
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KerryR
sage
Reged: 12/05/07
Posts: 324
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I have and love the Cannon 12x36 IS. You'll be glad you got them.
I don't binoview on my SCT, but, I think you'll be missing the wide FOV. I think you really have to decide what's important... I don't think you can get both wide views and optical precision without spending a LOT of money...
Kerry
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Kerry
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Muffin Research
scholastic sledgehammer
Reged: 09/28/07
Posts: 805
Loc: Belgium
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Joe going over your list you already got the lot...
So what do you think? BT's OR SCT with Bino's (no mentioning of Ethos please  )
Kerry, good to hear about the 12x36IS seemed like the largest smallest hands on binocular with IS technology and i'm expecting a lot from this, the jitter and movement is what annoyed me about tri-pod free bino viewing.
For viewing with mount, yes those BT's are in my head but I try to reason them out
Those BT 80's if I can just pick them up and put them outside mounted, like I do with the C8 that's going to be a supersweet instrument.
but then again being able to look at Planets at Mags in between 100x and 300x that's going to be awesome as well.
and most clusters and nebula are going to be quite a sight.
and it will make stereoviewing possible with two scopes if I opt for the binoview. however ease of use will still be with the Giant Binoculars and as you said before this comes to the choice of do it all instrument vs just great low power views....
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Quelle Horreur!
Requime pour un Twister.
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Joe Ogiba
Post Laureate
Reged: 02/14/02
Posts: 3362
Loc: NJ USA
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Quote:
So what do you think? BT's OR SCT with Bino's
Both, BT's up to 40x and SCT w/ binoviewer above 40x.
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Pentax PF-80ED
Meade 102ED APO
Orion EON 72
120ST
Apex 127
C6 XLT
CR150
C9.25
XT10
Zeiss 7x42 FL
Canon 10x42L IS WP
15x50 IS
12x36 IS II
Garrett Optical 28x110 HD-WP Signature Series
Oberwerk BT-80 45
Apogee RA-88-SA
Denk II Power x Switch binoviewer w/13mm Ethos, 20mm Pentax XW's, 20mm Widescan III's.
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