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EdZ
Professor EdZ


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Binoviewers and Binoculars new
      #1390448 - 01/29/07 06:44 AM

These questions were asked in another thread.

Quote:

I wonder what it would be like to use a 120mm or 150mm short FL Achro with a bino-viewer.

In some ways it's all the advantages of a scope and giant binocs at the same time, though binoviewers do have some inherent disadvantages of their own

that is true, focus issues and more light loss. I wonder what a 150mm F5 w/BV would equate to in the binocular world.





Just to dispense with a few of the misconceptions that I've been reading so far in other threads:

Merging images in a binoviewer is not an issue. Two self-centering collet models required no adjustment at all. put in the eyepieces and view. One model with 3 screws to hold the eyepieces, similar to a finder scope, required about 30 seconds to merge the images.

Focusing took a little longer. Two BVs had both right and left dipoter adjustment to fine tune after focusing with the scope. However, temperatures around 10° made both focusers very stiff, so fine adjustment was difficult. Not unlike some binoculars in extreme cold temps. Another BV does not have diopter adjustment. After focusing with the scope to one eye, the other eyepiece lock is loosened and move very slightly out, then relocked.

I can't yet report on actual light transmission. However, I can state this. There is only so many square mm of light entering the scope. A binoviewer is a beam splitter. If it is working as it is designed to do, and at 100% efficiency, then it splits the light exactly one half to each eye. The net result is that the light delivered to each eye can be NO MORE than ONE HALF the total light gathered. That's the same as one half the area of the aperture. Some binoviewers do not split the light equally, and if so, this will diminish the binocular summation effect.

I'll use all nominal measures here, but follow along, this all works.

A 150mm scope has an area of 150x150 = 22500 sqmm
That gets split, half to each eye, therefore 22500/2 = 11250
That's like getting light from two 106mm apertures
So a 150mm scope with a binoviewer is like a 106mm binocular.

A 120mm scope has an area of 120x120 = 14400 sqmm
That gets split, half to each eye, therefore 14400/2 = 7200
That's like getting light from two 85mm apertures
So a 120mm scope with a binoviewer is like a 85mm binocular.

Depending on the binoviewer you buy, some have larger clear aperture than others. Some are 20mm clear aperture right at the field stop where the eyepiece sits, some are 26mm clear aperture.

Binoviewers come with one or more small barlow-like attachments called an OCS, (optical corrector system) or an optical nose piece, sometimes necessary to allow them to reach focus in a refractor or a newtonian. The OCS attachments are generally about 2.0x. The optical nosepiece is a 0.8x reducer. The multiplier is not necessary in an SCT scope. A BV with no corrector, used this way only in an SCT, results in approximately 1.25x the base focal length. The optical nose reducer is for SCT scopes, in this case approx 0.8x.

Advantages of the binoviewer is the ability to vary magnification to suit your needs and hence view in binovision. Not to mention, the BV sits in a 90° diagonal, so viewing comfort level is improved. The binoviewer can be moved from one scope to another, so the comfort of binovision can be moved around.

Binoviewers maintain the resolution of the source optics. So while the light from a 150mm scope will be split to two eyes, the BV will maintain the resolution of the 150mm scope.

Now for one of the most drastic disadvantages of the binoviewer. FOV
The CA (clear aperture) of the binoviewer restricts the field stop of the eyepiece. Not only does that somewhat vignette the light that gets thru, meaning any eyepiece with a wider field stop than the clear aperture will have vignette in the outer edges of the field, but, more importantly, the CA restricts the field of view. You cannot get a larger field of view by using an eyepiece that has a field stop much wider than the CA (clear aperture) of the binoviewer.

The less expensive binoviewers (up to $200) on the market today mostly have a 20mm CA (clear aperture of prism). How much that 20mm CA restriction will affect the eyepiece you select depends it seems on the placement of the field stop in the eyepiece housing. I've had numerous samples test out to equivalent of 21 - 23mm field stop. That is the field stop diameter in a 23-25mm plossl eyepiece. Therefore the widest possible field of view that you can get with a 20mm CA binoviewer would be that which you would get with a 21-23 mm field stop. That would be near the equivalent of a 24-25mm plossl eyepiece, a wide field Afov 60° 20mm eyepiece, or a 68° 18mm, or an 84° afov 15mm. All of these eyepieces would give the near the widest possible TFOV in a 20mm CA binoviewer. No eyepiece with a larger field stop will give it's equivalent wider field of view.

If you select a refractor like a short tube 150mm that is f/5, you start out with a 750mm focal length. The refractor OCS doubles that to 1500mm. In a 20mm CA binoviewer, an eyepiece with ~21mm FS, a 23mm plossl, will give you near the widest low power FOV. That's 65x with 0.8° Tfov. A 30mm plossl, even though it has a 26mm field stop (25% wider), gives you 50x with just about 0.85° TFOV. That 30mm eyepiece views as if it has a 40° Apparent FOV.

If you select a refractor like a short tube 120mm that is f/5, you start out with a 600mm focal length. The refractor OCS doubles that to 1200mm. In a 20mm CA binoviewer, A 23mm plossl will give you the widest low power FOV. That's 52x with a 1.0° Tfov. The 30mm eyepiece would give you 40x, with same 1.0° TFOV, so if you used a 30mm plossl, the clear CA would close the eyepiece Afov down to near Afov=40°. A 40mm eyepiece would give you 30x and a 4mm exit pupil, but it would result in a 30° Afov. Talk about looking thru a straw!!! The only way to get a wider field of view is to go with a more expensive binoviewer that has a larger CA.

With stock 20mm CA binoviewers as sold which means it comes with a 2.0x OCS (for refractors), The lowest power widest orthoscopic (40°Afov) view you could get would be approximately:
150mm f/5 scope – 50x @ 0.8° max Tfov for equivalent 50x106 binocular
120mm f/5 scope - 40x @ 1.0° max Tfov for equivalent 40x85 binocular

There are other accessory OCS units for refractors sold aftermarket that have some lower multiplier factors. These will help reduce the effective total focal length, and because focal length would be shorter, it would help increase maximum field of view.

For max FOV, you can do slightly better than the refractors by using equal diameters of f/10 SCTs, but only when using the 0.8x reducer optical nosepiece. Once again, total FOV will be restricted by the Binoviewer CA. In my 5" SCT, using a 26mm CA binoviewer with 0.8x optical nose, I was able to get a maximum TFOV of 1.4° using a 26mm/60° eyepiece with an eyepiece with a 27.3mm field stop, about the maximum diameter field stop for any 1.25" eyepiece. Some 1.25" eyepieces achieve a slightly wider field of view by eliminating the field stop ring, which generally results in a poor darkened non-descript edge of view.

My 5" C5 SCT, after subtracting for central obstruction, is the equivalent of an 81mm bincular. So the best possible widest field I can get, even with a higher quality 26mmCA binoviewer/0.8x opnose, is by using my 26mm Meade/60° eyepiece with a 27.3mm field stop. This gives equivalent to 44x81 with 1.4° TFOV. Using the lower quality 20mmCA binoviewer/1.25x (no OCS) the best I can get is 64x81 with a 0.8° TFOV. In both cases I can use a longer focal length eyepiece for slightly lower power, but cannot get any wider FOV.

The new Celestron C6 SCT with diagonal would operate about f/11. With binovewer and the Denkmeier 0.8x optical nose it would have a focal length near 1320mm. With a 30mm eyepiece you could get a lowest power of 44x. Since Denkmeier has a CA (clear aperture) of 26mm, you would be able to get a widest FOV of about 1.2° in this 6" scope. After subtracting the light loss for the central obstruction (50mm dia), the C6 would give a maximum widest field equivalent of a 100mm binocular at 1.2° at 44x. You could lower the power by using a 32mm or 40mm eyepiece, but you cannot get a wider FOV.

With stock 20mm CA binoviewers as sold for SCT, no OCS needed, (the BV operates at ~1.25x), The lowest power widest orthoscopic (40°Afov) view you could get would be approximately:
150mm f/10 SCT – 65x @ 0.7° max Tfov for equivalent 65x100 binocular
125mm f/10 SCT - 50x @ 0.9° max Tfov for equivalent 50x81 binocular

For comparison, the BT100/45° gets 43x using 14mm Radians. The TFOV is 1.4°. The advantage here is you could put a serious widefield eyepiece in the BT100 and get wider fields. The 13mm Hyperion/68° in the BT100/45 would give you 46x with a 1.5° TFOV. And the clear aperture of the prisms in the BT100 will not vignette these wide field eyepieces.

You can use longer focal length eyepieces like 26mm or 30mm in a 20mm CA binoviewer. You will get lower magnification. But you will not get an equivalent wider field of view that matches the field stop and the edges of the exit pupil will be vignetted. If you want a wider fov, you need to buy a more expensive binoviewer with a 26mm clear aperture like the Denkmeier or similar 26mm CA.

You will never get the field of view that you get from a binocular of equivalent aperture. BUT, a fixed power binocular will never get the higher magnifications available in the binoviewer.

Binocular telescopes, since they maintain the shorter focal length, do provide the ability to increase magnification while maintaining wider FOV. However, the fast binocular optics generally introduce greater aberrations. The BV allows the use of longer effective focal lengths and reduces those aberrations, but restricts FOV.

edz


--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21

Edited by EdZ (05/24/07 04:33 PM)


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Tony Flanders
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Re: Binoviewers and Binoculars new [Re: EdZ]
      #1390775 - 01/29/07 11:28 AM

Quote:

So a 120mm scope with a binoviewer is like a 85mm binocular.




Yes, but with one critical caveat -- the binoviewer's exit pupils are 1.4 times wider than the binoculars. That's why binoviewers can never take the place of binoculars for viewing at ultra-low magnifications.

Consider the classic 7x50 binoculars. They have 7mm exit pupils, matching the nominal size of most young people's eyes. To achieve the same light throughput with a binoviewer, you'd need a 70mm scope. But a 70mm scope at 7x yields a 10mm exit pupil, much wider than *any* person's eyes can take in. For most people, a 70mm scope would actually deliver no more light at 7X than a 50mm scope. So the total light entering your eyes from a 7x70 scope with binoviewer is actually only half what you get from 7x50 binoculars.

Put another way, 50mm binoculars are better for viewing at 7X than any single-objective telescope can possibly be.

How about 10x50 binoculars versus a binoviewer on a 10x70 scope -- still assuming that your own pupils open to 7mm? The binoculars are *still* better, even in this case. That's because the outer part of most people's lenses has lousy optical quality. The reason your pupils close down in bright light isn't just to protect your retinas -- it's also to reduce the inherent optical aberrations of your eye, just as stopping down a 50mm f/1.4 photo lens to f/2 almost always yields far superior results.

At the opposite end of the magnification scale, binoviewers are actually superior to binoculars, for precisely the same reason. Most people find exit pupils smaller than 1mm fairly hard to handle, and many people run into serious problems with floaters. So a binoviewer that delivers two dim 1mm exit pupils is actually superior to binoculars that concentrate the same amount of light into 0.7mm exit pupils.

In this case, practice follows optical theory. Traditional binoculars are used almost exclusively for richest-field viewing, with exit pupils of 4mm and wider, where the benefits of having one objective per eye are greatest. And binoviewers are used primarily for viewing bright subjects at high magnifications.

--------------------
Tony Flanders

First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.


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KennyJ
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Re: Binoviewers and Binoculars new [Re: Tony Flanders]
      #1390848 - 01/29/07 12:17 PM

I must say a special thank you to EdZ and to Tony F.

Taken together , those two posts which preceded this are the most informative I've ever read about binoviewers v binoculars .

Just two weeks ago , it must have taken me about 8 HOURS of searching and reading snippets of similar information before arriving at my decision NOT to go ahead with getting a binoviewer to use with my either of my two spotting scopes !

Regards , Kenny

--------------------


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EdZ
Professor EdZ


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Re: Binoviewers and Binoculars new [Re: Tony Flanders]
      #1390862 - 01/29/07 12:26 PM

Quote:

Yes, but with one critical caveat -- the binoviewer's exit pupils are 1.4 times wider than the binoculars. That's why binoviewers can never take the place of binoculars for viewing at ultra-low magnifications.

Consider the classic 7x50 binoculars. They have 7mm exit pupils, matching the nominal size of most young people's eyes. To achieve the same light throughput with a binoviewer, you'd need a 70mm scope. But a 70mm scope at 7x yields a 10mm exit pupil, much wider than *any* person's eyes can take in. For most people, a 70mm scope would actually deliver no more light at 7X than a 50mm scope. So the total light entering your eyes from a 7x70 scope with binoviewer is actually only half what you get from 7x50 binoculars.

Put another way, 50mm binoculars are better for viewing at 7X than any single-objective telescope can possibly be.

How about 10x50 binoculars versus a binoviewer on a 10x70 scope -- still assuming that your own pupils open to 7mm? The binoculars are *still* better, even in this case. That's because the outer part of most people's lenses has lousy optical quality. The reason your pupils close down in bright light isn't just to protect your retinas -- it's also to reduce the inherent optical aberrations of your eye, just as stopping down a 50mm f/1.4 photo lens to f/2 almost always yields far superior results.






Not to worry, Tony. These kinds of (low power) comparisons are pretty much unachievable. They are totally outside the realm of a binoviewer.

Just sticking with your sample of a 70mm scope, let's say we had a 70mm scope that is f/5. That's a 350mm focal length. Now let's say we purchased the smallest aftermarket correcter available, a 1.25x OCS. That makes the focal length of the scope 438mm. Now, overlooking the fact that if you tried to get 7x with a 70mm scope it would be effectively more like a 50mm scope (which BTW would be the equivalent of 7x35 binoculars), to get 7x you would need a 62mm eyepiece! You would get a maximum 2.7° True FOV. Your 62mm eyepiece would have an effective 19° Afov. To get 10x you would need a 44mm eyepiece and the effective Afov would be 27°.

So lowest power you could get with that 70mm scope/BVx1.25, "with a reasonably normal orthoscopic 40° Afov" would be 15x. That would be a 30mm eyepiece and you'd get a 4.67mm exit pupil.

My two most used small refractors are WO 80/500 and TV 85/600. As it stands, the OCS that come with the BVs makes these 80/1000 and 85/1200. I could use my 40mm TV plossl for a lowest power of 25x and 30x. If I were to purchase that aftermarket 1.25x OCS, then with the same 40mm eyepiece I could get 16x and 19x. So it would take me a 40mm eyepiece to get a 5mm exit pupil. But due to light splitting I would end up with effectively 16x56 or 19x60. I would get a maximum 2.5° TFOV, so Afov would be 40° to 50°.

With stock 20mm CA binoviewers as sold, which means it comes with a 2.0x OCS, The lowest power widest orthoscopic (~40°Afov) view you could get would be approximately:
100mm f/5 scope – 33x @ 1.2° max Tfov for equivalent 33x71 binocular
80mm f/5 scope - 26x @ 1.6° max Tfov for equivalent 26x56 binocular
70mm f/5 scope - 23x @ 1.8° max Tfov for equivalent 23x49 binocular

The only reason I say orthoscopic views is this: Max Tfov does not change. You could get a wider Afov, but only at the expense of a higher power.

That's why these comparisons aren't even made to low power binoculars. Generally they are made to 60mm, 70mm 80mm or 100mm binoculars or the BT100 and usually in the 20x+, more like 30x to 40x low power range.

edz

--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21

Edited by EdZ (01/30/07 09:32 AM)


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Glassthrower
Vendor - Galactic Stone & Ironworks


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Re: Binoviewers and Binoculars new [Re: EdZ]
      #1390901 - 01/29/07 12:46 PM

Indeed, a very informative thread. Thanks EdZ and TonyF for sharing. If I needed any more convincing not to get a binoviewer, this provided it....I know that was not the intention of this thread, but given my viewing tastes/budget, I do not forsee a binoviewer entering my gear inventory any time in the near future.

The "last straw" in this respect was EdZ' revelation (to me) that the cheaper BV units only have 20mm clear aperture. Which essentially wastes any eyepiece with a stop larger than that......well, not "waste", perhaps I should say "not fully utilized".

Which begs the question - are there any BV units available that accept 2" eyepieces or have a CA>27mm? People with wide-set eyes might be able to use a 2" BV.

Regards,

MikeG

--------------------
Michael Gilmer - Member of the Meteoritical Society & Collector of Falling Stars.



☄ ⒼⒶⓁⒶⒸⓉⒾⒸ ⓈⓉⓄⓃⒺ ☞ www.galactic-stone.com


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EdZ
Professor EdZ


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Re: Binoviewers and Binoculars new [Re: Glassthrower]
      #1391051 - 01/29/07 02:17 PM

Quote:

Indeed,Which begs the question - are there any BV units available that accept 2" eyepieces or have a CA>27mm? People with wide-set eyes might be able to use a 2" BV.

Regards,

MikeG




Siebert Black Knight
Basic 40mm CA 2" BV sells for $1795.
Premium 45mm clear aperture 2" BV costs $2000.

Adjustable diopters extra, minimum $120 for one side.
2" to 1.25" adapters, cause you won't always use 2" eps. $60 each.
Optical Correctors extra. $159$ to $299.
No eyepieces included.

So total cost is $2200 to $2600, before eyepieces.


Current waiting list for both is 1 year.


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zdravko
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Re: Binoviewers and Binoculars new [Re: EdZ]
      #1391168 - 01/29/07 03:15 PM

I know I may be out of topic with these but.The best yet known way to me guys, is to get one of the Matsumotos EMS and a pair of OTAs of your choosing.I have been down this road how to get the widest binocular views and the high power views at the same time.I think it is better and cheaper than Astromechanica binocular telescopes.It has the flexibility of using 2" eyepices.Colimation is easy with a pair of tumb screws and can support as high magnification as yout telescopes can take.And if you compare it to the Siebert BV unit it is even cheaper if you don't count a pair of OTA-s that you have to buy.
Here is the link.
http://www.page.sannet.ne.jp/mazmoto/user-j.htm

--------------------
TMB 175/F8 APO on CGE
Astreya 102/F6,5 APO
Astreya 76/F6 APO
152/F5,9 BINOSCOPE in progress
120/F5 BINOSCOPE
76/F6 APO BINOSCOPE in progress
10X50 APM binocular
3-6mm N Zoom,9mm NT6, ETHOS 6,8,2X13,17mm,2X 22mm NT4,2X Meade SWA 24,5mm,2X BW optik 30mm
http://zdravko.spricer.com/


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Erik D
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Re: Binoviewers and Binoculars new [Re: zdravko]
      #1391196 - 01/29/07 03:32 PM

There is a company in the US offering Binoscopes with the EMS backs. I have seen them at scope shows. Any binoscope 127mm or larger with fork mount and tripod is massive. Anything less than 5 inch and the BT 100 45 deg complete with fork mount and tripod is much more cost effective. Costs aside, A 6 in F8 refractor takes some effort to set up and move around. A pair of them is more than twice that:

http://www.binoscope.com/bigbino.htm


Erik D


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Glassthrower
Vendor - Galactic Stone & Ironworks


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Re: Binoviewers and Binoculars new [Re: Erik D]
      #1391248 - 01/29/07 04:01 PM

Quote:


Siebert Black Knight
Basic 40mm CA 2" BV sells for $1795.
Premium 45mm clear aperture 2" BV costs $2000.

Adjustable diopters extra, minimum $120 for one side.
2" to 1.25" adapters, cause you won't always use 2" eps. $60 each.
Optical Correctors extra. $159$ to $299.
No eyepieces included.

So total cost is $2200 to $2600, before eyepieces.


Current waiting list for both is 1 year.





Wow. Ouch.

But, I bet the stereo views through a pair of 2" eyepieces would be amazing.

Another issue that I can see with a 2" binoviewer is weight. a 2" diagonal, 2" binoviewer body and two 2" eyepieces is a lot of weight. (not counting nosepieces, filters, etc) Such a load would require a premium, heavy-duty focuser that would also add a substantial amount of $$$ to the equation.

Regards and clear skies,

MikeG

--------------------
Michael Gilmer - Member of the Meteoritical Society & Collector of Falling Stars.



☄ ⒼⒶⓁⒶⒸⓉⒾⒸ ⓈⓉⓄⓃⒺ ☞ www.galactic-stone.com


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EdZ
Professor EdZ


Reged: 02/15/02
Posts: 18806
Loc: Cumberland, R I , USA42N71.4W
Re: Binoviewers and Binoculars new [Re: Glassthrower]
      #1391341 - 01/29/07 05:08 PM

Quote:

Another issue that I can see with a 2" binoviewer is weight. a 2" diagonal, 2" binoviewer body and two 2" eyepieces is a lot of weight. (not counting nosepieces, filters, etc) Such a load would require a premium, heavy-duty focuser that would also add a substantial amount of $$$ to the equation.

Regards and clear skies,

MikeG




Well, even cheap 2" eyepieces go, what, maybe $150 a piece. So, someone who is spending close to $3000 for a BV with one pair of eyepieces to hang off the back end of a scope, whether it be a SCT (which has no focuser issues cause it's the mirror that moves) or a refractor, has already invested a considerable sum in the rest of the equipment and likely already has whatever is needed to handle the job.

But for the simple reason that the 2"BV puts us into a realm that is in the cost stratosphere, I'll restrict the remainder of my discussion to 1.25" binoviewers. These make for a reasonable comparison to the binoculars and binocular telescopes that we use.

Summing up some of the data from my posts above:

With stock 20mm CA binoviewers as sold for SCT, no OCS needed, (the BV operates at ~1.25x), The lowest power widest reasonable orthoscopic (40°Afov) view you could get would be:
203mm f/10 SCT – 88x @ 0.5° max Tfov for equivalent 88x135 binocular
150mm f/10 SCT – 65x @ 0.65° max Tfov for equivalent 65x100 binocular
125mm f/10 SCT - 54x @ 0.9° max Tfov for equivalent 54x81 binocular

With stock 20mm CA binoviewers as sold, which means it comes with a 2.0x OCS for refractor, The lowest power widest orthoscopic (40°Afov) view you could get would be:
150mm f/5 scope – 50x @ 0.8° max Tfov for equivalent 50x106 binocular
120mm f/5 scope - 40x @ 1.0° max Tfov for equivalent 40x85 binocular
100mm f/5 scope – 33x @ 1.2° max Tfov for equivalent 33x71 binocular
80mm f/5 scope - 26x @ 1.6° max Tfov for equivalent 26x56 binocular
70mm f/5 scope - 23x @ 1.8° max Tfov for equivalent 23x49 binocular

All three binoviewers I've tested so far in the SCT have produced a magnification factor of 1.23x to 1.26x, with most eyepieces falling at 1.24x or 1.25x. All testing is done by drift timing a star and correcting time by cosine declination.

Several answers to get wider field of view:
Purchase an aftermarket refractor OCS of 1.8x, 1.6x or 1.25x, instead of the stock 2.0x OCS
Purchase a binoviewer that has a larger 26mm clear aperture.
Use an optical corrector/reducer if one is available for your brand BV.

With an available aftermarket 1.25x in place of the stock 2.0x OCS in a stock 20mm CA binoviewers, The lowest power widest orthoscopic (40°Afov) view you could get would be:
150mm f/5 scope – 30x @ 1.3° max Tfov for equivalent 30x106 binocular
120mm f/5 scope - 25x @ 1.6° max Tfov for equivalent 25x85 binocular
100mm f/5 scope – 21x @ 1.9° max Tfov for equivalent 21x71 binocular
80mm f/5 scope - 16x @ 2.5° max Tfov for equivalent 16x56 binocular
70mm f/5 scope - 15x @ 2.9° max Tfov for equivalent 15x49 binocular


Still none of these options will get you as large a field of view as an f/6 100mm or f/6.5 80mm binocular telescope. I've already shown what you can get with the 1.25x OCS. If you also purchase a 26mm clear aperture binoviewer, maximum TFOV will increase by about 15%-20%. That's still no more than the BT100.

And keep in mind, these will all look like orthoscopic views thru the binoviewer. The only reason I say orthoscopic views is this: For a given scope focal length, Max Tfov does not change. You could use an eyepiece that would give the look of a wider Afov, but only at the expense of a higher power.

For comparison to the above Binoviewer True FOV:
the Oberwerk BT100 (straight thru = 620mm F) has been tested often and found to have:
with 26mm/60° Meade 5k plossls, 24x @ 2.5° Tfov
with 26mm TV plossls, 24x @ 2.1° Tfov
with 24.5mm Obie WA, 25x @ 2.5° Tfov
with 20mm TV plossls, 31x @ 1.6° Tfov
with 18mm Meade 4k SWA, 34x @ 1.8° Tfov
with 14mm TV radians, 44x @ 1.4° Tfov
with 10mm Obie WA, 62x @ 1.0° Tfov

some other eyepieces would give these FOV in the BT100:
A 24mm Panoptic/65° in the BT100 gives 26x @ 2.5°
A 13mm Hyperion/68° in the BT100 gives 48x @ 1.4°
A 9mm Nagler/82° in the BT100 gives 69x @ 1.2°
A 7mm Nagler/82° in the BT100 gives 88x @ 0.9°

Given some of the reports we have here about the variety of binocular telescopes with interchangable eyepieces avaiable to them, certainly there are valid considerations to be weighed for the viability of variable powered binoculars. But, I agree with Tony's comment, in the extreme of high powered magnifications, binoviewers have their place.

I will say though that I have spent some very enjoyable time viewing, mostly with the 5" SCT, but also with an 80mm refractor. Most of my observing has been at 44x (equiv to 44x81), 73x (for 73x81) and 38x in the 80mm for equiv of 38x56. There is definitely a place in my stable for these as a useful and enjoyable tool.

Also, found my notes indicating I spent an evening using 26mm TV plossls at 119x. Here's some notes:
Alnitak, zeta Ori ssen secondary south following by about 10-15°.
M42 impressive, Trapezuim is large, no E star.
NGC2392, Eskimo neb, seen quite large.
M35 stunning, fills entire fov, missed 2158.
Saturn, Cassini in and out of view, see 2 faint moons.

edz

--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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KennyJ
The British Flash


Reged: 04/27/03
Posts: 20139
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Re: Binoviewers and Binoculars new [Re: EdZ]
      #1391347 - 01/29/07 05:17 PM

< There is definitely a place in my stable for these as a useful and enjoyable tool.
>

Ed ,

The way you're heading , there will be no room left for a horse in YOUR stable soon ! :-)

Thanks for continuing to share such rare and valuable information with us all here .

Regards , Kenny

--------------------


Milton Wilcox R.I.P






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camvan
Carpal Tunnel


Reged: 05/02/05
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Loc: British Columbia
Re: Binoviewers and Binoculars new [Re: KennyJ]
      #1391605 - 01/29/07 07:38 PM

forgive my ignorance guys...but is there any way to compensate for the focus of a BV without having to use the OCS?

and on top of that, really forgive my ignorance, but why would you need to add so much extra power with an OCS for something that only extends maybe 6" off your focuser than an eyepiece? what is it about the splitting that means you have to magnify?

and being that where we are with our bino's our 8" Dob's are not far behind, I can only imagine what kinda views we'd get with it. Edz, lacking the optical mathematics, how do I translate the 20000 (200x200= 40000. 40000/2 = 20000) into binocular aperture?

edit - how cad of me! Edz, thank you sooo much for this! I hope you'll put it at the top of the 'Best Of' section and I am honestly thinking perhaps an honorable mention of this should be put into the BV forum so they can see what we're talking about here in binoland!

--------------------
Cameron

"Aperture can only be replaced by even more aperture. Dark transparent skies cannot be replaced by anything else." - Stathis Kafalis

Intes MN66
Meade SN8
handfull of cheap ep's

Edited by camvan (01/29/07 07:52 PM)


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milt
professor emeritus


Reged: 09/13/04
Posts: 603
Loc: Arizona
Re: Binoviewers and Binoculars new [Re: camvan]
      #1391781 - 01/29/07 08:53 PM

is there any way to compensate for the focus of a BV without having to use the OCS?

You can shorten the in-focus requirement by using a prism diagonal in place of a mirror diagonal. Beyond that you have to shorten the OTA tube.

what is it about the splitting that means you have to magnify?

The total effective path length through all the prisms of a binoviewer is 100mm or more. If you have a light cone converging at a 4:1 ratio (i.e. f/4) then you have to place the entrance of the binoviewer where the light cone is 25mm in diameter in order for it to finish converging at the exits 100mm (4x25) behind.

But what if you only have enough in-focus to reach a point in the light cone where it is 10mm in diameter at the binoviewer entrance? Then the focal plane will end up somewhere in the middle of your binoviewer. So you use a Barlow to change the light cone to f/10, which gets you back to the needed 100mm path length (10x10). Changing from f/4 to f/10 is a magnification of 2.5x, typical for these setups.

The following two paragraphs have been corrected

The OCS systems are more complex. They start with a strong negative lens stuck up inside the focuser tube at the 25mm light cone diameter point for the above example. This diverges the light cone to a very slow f/ratio that brings it back to the binoviewer entrance little converged. Then, right at the entrance of the binoviewer they follow with a strong positive lens to bring it back to close to the original f/ratio as if the binoviewer itself had been moved up inside the focuser tube.

The OCS is a bit of a kludge that adds more glass and places more stringent optical requirements on the binoviewer prisms due to the steep light cone passing through them. IMO it is preferable to use a Barlow or a slower scope with a shortened tube and lots of focuser out-focus range for when the binoviewer is not being used.

I hope this helps.

Edited by milt (01/30/07 08:07 AM)


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camvan
Carpal Tunnel


Reged: 05/02/05
Posts: 2142
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Re: Binoviewers and Binoculars new [Re: milt]
      #1391793 - 01/29/07 08:58 PM

it helps some Milt, thanks. so I take it you can't do anything quite as simple as having a focuser with long travel to compensate and achieve focus?

--------------------
Cameron

"Aperture can only be replaced by even more aperture. Dark transparent skies cannot be replaced by anything else." - Stathis Kafalis

Intes MN66
Meade SN8
handfull of cheap ep's


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RichardHK
sage


Reged: 11/25/06
Posts: 396
Loc: Hong Kong
Re: Binoviewers and Binoculars new [Re: camvan]
      #1391901 - 01/29/07 09:50 PM

Time to make this thread a sticky Mr Moderator!

Excellent, first class professional information.

--------------------
Richard Entwistle, Hong Kong
ETX-125, SV90TBV, & PST scopes
Canon 15x50-IS & Fujinon 7x50 bins


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EdZ
Professor EdZ


Reged: 02/15/02
Posts: 18806
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Re: Binoviewers and Binoculars [Re: RichardHK]
      #1391967 - 01/29/07 10:20 PM

what we do in this forum, rather than make threads sticky, is add links to our library under the proper index card. For the time being I can add a link to this one under binocular vision.

edz

--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21


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milt
professor emeritus


Reged: 09/13/04
Posts: 603
Loc: Arizona
Re: Binoviewers and Binoculars new [Re: camvan]
      #1392173 - 01/30/07 12:16 AM

you can't do anything quite as simple as having a focuser with long travel to compensate and achieve focus?

That's exactly what you do, but you have to start with a shorter tube so when the fouser is racked in you can reach focus. Then when you are not using the binoviewer, you rack the focuser back out and/or add an extension tube.

Milt


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camvan
Carpal Tunnel


Reged: 05/02/05
Posts: 2142
Loc: British Columbia
Re: Binoviewers and Binoculars new [Re: milt]
      #1392224 - 01/30/07 12:54 AM

when you mean shorter tube, are you meaning something shorter than F5, or are you referring to something else? a low-profile focuser doesn't make sense, being you need 'more' travel to get the allowable focus necessary.

but wait...I'm going to guess having the longer focus will in effect create that 0.8/1.25/2.0x effect?

please forgive me if I'm a bit slow adapting to this. math was NEVER my good point and mechanics of any sort require math, so right now I feel like I'm swimming in mud

--------------------
Cameron

"Aperture can only be replaced by even more aperture. Dark transparent skies cannot be replaced by anything else." - Stathis Kafalis

Intes MN66
Meade SN8
handfull of cheap ep's


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milt
professor emeritus


Reged: 09/13/04
Posts: 603
Loc: Arizona
Re: Binoviewers and Binoculars new [Re: camvan]
      #1392533 - 01/30/07 08:15 AM

when you mean shorter tube, are you meaning something shorter than F5, or are you referring to something else?

Shorter tube means you take a tube and cut 3 or 4 inches off it.

Note I have corrected my original post regarding the operation of the OCS - brain fade on my part.

Milt


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Joe Ogiba
Postmaster


Reged: 02/14/02
Posts: 5446
Loc: NJ USA
Re: Binoviewers and Binoculars new [Re: milt]
      #1392780 - 01/30/07 11:08 AM

Some scopes come with short tubes for binoviewer use like the TMB 115 F7 Lightweight APO Refractor and you just add the extender when not using a binoviewer.

--------------------
Pentax PF-80ED,Meade 102ED APO,Orion EON 72,120ST
Apex 127,C6 XLT,CR150,C9.25,XT10 ,Celestron Regal 100 F-ED, CT152
Zeiss 7x42 FL,Canon 10x42L IS WP,15x50 IS
12x36 IS II , Pentax 8x32 ED
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.
21mm Ethos,17mm Ethos, 22mm Nagler, 40mm Pentax XW, 14mm Pentax XL, 5.2mm Pentax XL, 8-24mm Pentax XL Zoom, 31mm Axiom LX
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