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Rich V.
Pooh-Bah
Reged: 01/02/05
Posts: 1394
Loc: Carson Valley, Nevada, USA
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I've really enjoyed the discussion on this forum about effective aperture and prism size as it relates to binocular design. There are some really knowledgeable members here and it has been very interesting.
There's been quite a bit of interest lately in the larger sized "binoscope" type binoculars that have an interchangeable eyepiece feature. Miyauchi introduced interchangeable eyepiece astro binoculars years ago but they were relatively expensive and required proprietary eyepieces. Over the last decade there have been a few binoculars introduced that allow the use of the standard 1-1/4" telescope eyepieces. The most successful of these has probably been the Oberwerk BT100. More lately, there have been offerings of copys of the Miyauchi design. They have been modified to use the standard 1-1/4" eyepieces, attempting to make them more appealing to astronomers who are already conscious of the large variety of 1-1/4" eyepieces available to them.
Glenn's "True aperture? A Quick Way to Measure!" thread has brought to light that many of these interchangeble 1-1/4" eyepiece binoculars are actually operating as less than their full aperture, though. The big Oberwerk BT100s appear to actually be operating around 92mm and the GT100/45s appear similar based on limited reports. Only the Miyauchis with their dreaded non-standard eyepieces seem to be operating at their full claimed aperture. This has got me asking the question, WHY?
Though I have no experience in binocular design like Glenn, this has still caused me to wonder if a compromise had to be made so that the 1-1/4" eyepieces could be useable in these binoculars. It could explain why these binoculars have smaller effective apertures.
In the Miyauchis, the eyepeices are designed so that their focal point is very close to the last prism face of the binocular. The light cone extending past the last prism face is kept quite short because of this and the prism cluster can be kept as far from the objective as possible to prevent vignette.
Now, take an 1-1/4" eyepiece and try to get it to focus in the Miyauchi- it won't. It needs the light cone to protrude further back from the prism face; the eyepiece can't be racked in close enough to the prism because of it's longer design. This can be remedied by moving the prism cluster closer to the objective but then the light cone could be vignetted because the front of the prism is now moved into a wider portion of the light cone. Another remedy could be to lengthen the focal length of the objective and leave the prism cluster where it is, say from 500mm (Miyauchi) to 530mm (United Optics). Now the light cone protrudes 30mm further back from the last prism face and a 1-1/4" eyepiece will come to focus but the light cone is now wider at the prism entry point and could again be vignetted. The same goes for the BT100; it has larger prisms than the Miyauchi copies but it's prisms still must be moved up the light path to provide the back focus necessary for the 1-1/4" eyepieces.
So, while 1-1/4" eyepieces present no problems for telescope use, could their very design be causing them to be less than optimal solutions for binoculars? Glenn uses telescope eyepieces in his home made binoculars so what's he doing in his design that allows them to work well? I would guess the shorter prism light path provided by not using prisms for IPD adjustment might be an answer.
I'm more curious than ever! Any thoughts?
Rich V
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Binoculars:
33/50/75/150x100 Saturn III, 16x70 FMT-SX, 10x50 PCF-V, 10x43 DCF-SP, 10x35 E2, 7x35 E, 8x30 E2, 7x26 Custom, 8x23AS Diplomat, 8x23 Travelite
Scopes:
C9.25, 6" f8 reflector, SV80S
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milt
professor emeritus
Reged: 09/13/04
Posts: 555
Loc: Arizona
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Rich, I think you are correct that minimizing prism size drives its position well back in the light path. Glenn knows what he's doing and undoubtedly sized the prisms for where he put them.
The focal planes of some older eyepieces, like the Nagler Type 1's I used for my Kowa, are farther forward. However I still had to have the barrels machined to reach focus.
Milt
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KennyJ
Reged: 04/27/03
Posts: 12901
Loc: Lancashire UK
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Interesting post , Rich .
Glenn impresses me so much as one so rarely knowledgable with regard to the actual designs and workings of binoculars that I'd be very surprised if he can't captivate us even more with answers to your queries !
Kenny
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beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 506
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Rich V.
As you know, I currently own the GBT 100/45 and have no problem with most of the 1 1/4 ep's that I own. Can I provide you with any measurements that will help with this thread.
Beachchairbill
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14731
Loc: Cumberland, R I , USA42N71.4W
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Well, it's not the prisms that make the BT100 smaller than actual aperture. There is a baffle part way down the tube that cuts the aperture.
The prism apertures in the BT100 are IIRC about 32mm in diameter, larger than the largest possible 1.25" eyepiece field stop, which is why eyepieces are so well illiuminated in the BT100.
So there are no problems with ANY 1.25" eyepices in the BT100. In fact even if the baffle were larger to allow a full aperture, the prisms would be able to handle the light cone and still provide near maximum illumination.
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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charen
Pooh-Bah
Reged: 05/28/05
Posts: 1083
Loc: New Zealand
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I wrote to Kevin at Oberwerk about the concerns re. the effective aperture of my 100-45 which I measured at 92mm.
I was under the strong impression it was also the baffle which restricts the incoming light cone. [Presumably this reduces CA.]
His reply
Hi Chris,
Yes, I noticed the thread on Cloudy Nights. I need to do a little more testing, but my preliminary tests with the flashlight backed away from the EP, the circle is around 92mm. But with the LED flashlight up against the EP, I get a full 100mm from the BT-100 and about 96mm from the BT-100-45. Under-size prisms are usually the cause of a binocular operating at less than full aperture, but these models have by far the largest prisms I've ever seen in a 100mm binocular. I'm wondering if the inside diameter of the 1.25" eyepiece barrel might be the limiting factor. Will research this a little more thoroughly when I have more time, right now am swamped with new orders to prepare and ship...
Regards,
Kevin
Chris
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35 binos.
80mm Cat.
WO66ED
SV NH 80mm / EQ3
Meade 8in.LX90
Skywatcher Equinox ED120 / Goto HEQ5.
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Rich V.
Pooh-Bah
Reged: 01/02/05
Posts: 1394
Loc: Carson Valley, Nevada, USA
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Ed, what do you think is the main reason for this baffle?
If the eyepiece would still be well illuminated without it, is the baffle there only to help remove objective aberrations? Essentially they're stopping a 100mm f6 down to a 92mm f6.5?
Thanks,
Rich V
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Binoculars:
33/50/75/150x100 Saturn III, 16x70 FMT-SX, 10x50 PCF-V, 10x43 DCF-SP, 10x35 E2, 7x35 E, 8x30 E2, 7x26 Custom, 8x23AS Diplomat, 8x23 Travelite
Scopes:
C9.25, 6" f8 reflector, SV80S
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KennyJ
Reged: 04/27/03
Posts: 12901
Loc: Lancashire UK
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Rich ,
Given it's probably the oldest trick in the book of optics , I would be very surprised if the reason was ANYTHING OTHER than what you suggested .
Kenny
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skysurfer
member
Reged: 10/05/09
Posts: 17
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When the binocular has not enough backfocus, you can put a barlow between the EP and the binucular. This guy did the same with using a Canon supertele as a telescope.
http://www.samirkharusi.net/televue_canon.html
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Two eyes (5-7mm in full darkness)
TS 15x70mm binos
TeleVue Genesis 100mm f=500mm
Orion XT-10i 250mm f=1200mm
Canon EOS 40 w/85mm f/1.8, 50mm macro f/2,5
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orlandog
member
Reged: 01/10/08
Posts: 33
Loc: wilmington, north carolina
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Rich V.
You wrote:
“…So, while 1-1/4" eyepieces present no problems for telescope use, could their very design be causing them to be less than optimal solutions for binoculars? Glenn uses telescope eyepieces in his home made binoculars so what's he doing in his design that allows them to work well? I would guess the shorter prism light path provided by not using prisms for IPD adjustment might be an answer.”
I cannot speak for Glenn however, interchangeable 1-1/4" eyepieces are being used in a homemade bino. A Ray Sketch can be seen at:
http://picasaweb.google.com/hrrrose/Bino3#
I believe, this confirms that all is well
The design is:
objFocalLen(mm) =600.0; objAperture(mm) =70.0
eyeFocalLen(mm) =40.0; eyeFieldStopAperture(mm) =27.5
prismOpticalLen(mm) =110.0; prismAperture(mm) =25.4
rearPrismToFieldStopLen(mm) =130.0;
objective to prism entrance face(mm) =360.0;
mag =15.0; exitPupil(mm) =4.7; TFOV(degree) =2.6;
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Rich V.
Pooh-Bah
Reged: 01/02/05
Posts: 1394
Loc: Carson Valley, Nevada, USA
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Orlandog, I think the added complication with the 45° "BT" type binoculars is their having fixed rather than hinged OTAs. This forces the use of an additional prism to provide IPD adjustments, further increasing the portion of the light path that must pass through prisms. In my mind, the longer the prism light path, the greater the chance of vignetting the light cone.
I'm thinking that the Miyauchi-style design that uses a compact prism cluster could be mostly to blame for any prism vignette of the light cone of the "clone" models. Perhaps this is a design compromise necessary to make a 13# 100mm BT rather than a 26# one. Again, perhaps Miyauchi realized that they needed a proprietary eyepiece to get the most out of their design. The modification of Miya's design to make use of 1-1/4" eyepieces may have some limitations, a couple of them being reduced effective aperture and very close tolerances regarding prism edges. Even in the longish f7.5 Miya Saturns I see that the prism edges are quite close to the outside edge of the exit pupil but do not intrude.
Since no one I'm aware of has ray traced a Miyauchi let alone one of the UO BTs we really don't have enough information to make any real conclusions.
BTW, Beachchair Bill, I'd still like to hear what other GT100/45 owners measure regarding effective aperture using Glenn's method; it would be interesting to see if they get similar results to your measurements. As of now, your GT100 is the only data point. I still question why you have prism edge intrusion into the light path of only one side of your binocular. As was discussd before, that suggests a QC issue, not one of optical design. Without more data points, though, who knows? My original premise may be totally incorrect! 
Thanks for your thoughts, all.
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Binoculars:
33/50/75/150x100 Saturn III, 16x70 FMT-SX, 10x50 PCF-V, 10x43 DCF-SP, 10x35 E2, 7x35 E, 8x30 E2, 7x26 Custom, 8x23AS Diplomat, 8x23 Travelite
Scopes:
C9.25, 6" f8 reflector, SV80S
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14731
Loc: Cumberland, R I , USA42N71.4W
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Quote:
In my mind, the longer the prism light path, the greater the chance of vignetting the light cone.
only if the longer prism path were to "push" the prism design front face closer to the objective lens and the front prism aperture did not increase in diamter. Otherwise, No.
Quote:
perhaps Miyauchi realized that they needed a proprietary eyepiece to get the most out of their design. The modification of Miya's design to make use of 1-1/4" eyepieces may have some limitations, a couple of them being reduced effective aperture...
Eyepieces do not have any implications that could reduce aperture. Eyepieces are completely behind the focal point and therefore have no effect on aperture.
The only possible implications would be if the eyepiece generated such a large exit pupil that the eye could not use it all, then the too large exit pupil created by the eyepiece could reduce aperture.
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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Rich V.
Pooh-Bah
Reged: 01/02/05
Posts: 1394
Loc: Carson Valley, Nevada, USA
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Quote:
Quote:
perhaps Miyauchi realized that they needed a proprietary eyepiece to get the most out of their design. The modification of Miya's design to make use of 1-1/4" eyepieces may have some limitations, a couple of them being reduced effective aperture...
Eyepieces do not have any implications that could reduce aperture. Eyepieces are completely behind the focal point and therefore have no effect on aperture.
Ed, my reference to reducing effective aperture was only as a possible by-product of pushing the prism cluster closer to the objective to provide more back focus for the 1-1/4" eyepieces. It's pretty clear that an eyepiece in and of itself would have no effect on effective aperture. 
Rich V
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Binoculars:
33/50/75/150x100 Saturn III, 16x70 FMT-SX, 10x50 PCF-V, 10x43 DCF-SP, 10x35 E2, 7x35 E, 8x30 E2, 7x26 Custom, 8x23AS Diplomat, 8x23 Travelite
Scopes:
C9.25, 6" f8 reflector, SV80S
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1283
Loc: Ottawa, Ontario, Canada
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The only commercial binoscope I've had the opportunity to examine is an RA88, and about three years ago at that. The following comments are based on memory.
This bino also has the position of focus very close behind the optical window protecting the rear prism face. It employs a Porro II group inside the rotatable housing and a pentaprism immediately preceding. Even though the pentaprism is a full inch (25mm) in face size, the aperture between it and the Porro II group is a smaller 18mm (or possibly 20mm?) in width. Additionally, a flat-edged 'bar' is strategiacally placed at one side of this aperture so as to overcome a problem pentas suffer from, namely a bright out-of-field but near-to-optial axis reflection. This bar is easily seen as a clipping of the pupil.
I've not done a geometrical ray trace (not having all necessary dimensions), but my impression is that it's the aforementioned 18mm (20mm?) aperture which is causing the aperture reduction. I'd have to re-examine the instrument in order to determine just why this aperture is of such size.Some possibilities, one or more of which may apply: the Porro II prisms have hardly larger face widths themselves; this aperture need not be allowed to get larger because of the penta's front face size and light cone geometry within the penta; or it's an additional means of overcoming the pentaprism's noted shortcoming.
By itself, a small-ish 18mm aperture shouldn't be a problem *IF* it's close to the prism's entrant aperture. But if it does instead lie significantly distant from the prism, it is more likely to clip the light cone too early.
A pentaprism has a not inconsiderable optical path length due to the 'figure 4' folding it accomplishes. I haven't done an accurate measurement/calculation, but I should think that per unit of clear aperture the glass path length is a factor of 3.5X larger. But this is still a bit less than that of a Porro, for which the minimum ratio is 4:1.
To first order, I'll provisionally say that the total glass path length in the RA88's prism system is near to seven inches (170-180mm), based on a 25mm face pentaprism and 21mm aperture Porro II group. The air block equivalent (due to the fact that the focus is shifted back by an additional amount equalling 1/3 the glass path length) is five inches. This would imply that the penta's front aperture of one inch should allow full on-axis illumination with an f/5 objective, *IF* the penta and Porro were actually in contact (which they're not) and the focal surface lay exactly at the rear face of the Porro (which it doesn't quite.) Given the extra spacing noted, it might require a slower f/5.5, or perhaps even closer to f/6 objective to retain full central illumination. And this assumes there is no restricting aperture, which the already considered 18mm one could well be.
I have no idea of the prism system in the Miyauchi and so can't comment.
The next paragraph is not concerned with vignetting or other illumination characteristics as introduced by eyepiece optics, but rather considers field illumination *at the field stop only*. In other words, how the prism system accommodates the ligh cone (primarily on-axis.)
Regarding location of focus and illumination of field, the eyepiece itself has no real bearing when all its optical elements are *behind* the field stop. In such case, the image-forming part of the system doesn't 'care' what comes after the stop, only what lies ahead of it. (Now, an eyepiece which *does* have lenses ahead of the stop will have some influence.) In most prism-based systems, and particularly when the objective f/ratio is on the short side, there are usually tight constraints placed on just how far back the focus can be placed for accessibility. Re-locating the focal surface by only 1cm or so can result in not insignificant aperture reduction.
I sure would like to have the chance to get back into the guts of an RA88 again; the previous time I was primarily concerned with collimating it. But I did note that it worked at an effective aperture of 78mm, and got most of the way toward determining why.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 967
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Those optical path lengths, for several prism types , are presnted well in both Mil Handbook 141, available on this site, and in the Warren Smith books, among other places.
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