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16 MM Ultra-Wide Angle - 1.25 inch
Design: 6-elements in 4 groups
Field of View: 80°
Eye Relief: 20 mm
Price: $44.95

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Right, 20mm eye relief and 80° AFOV in a 16mm 1.25" eyepiece for $44.95 . Poor Televue, they only could get 10mm eye relief with their 16mm Nagler.

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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.
17mm Ethos, 22mm Nagler, 40mm Pentax XW, 14mm Pentax XL, 5.2mm Pentax XL, 8-24mm Pentax XL Zoom

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16 MM Ultra-Wide Angle - 1.25 inch
Design: 6-elements in 4 groups
Field of View: 80°
Eye Relief: 20 mm
Price: $44.95

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Right, 20mm eye relief and 80° AFOV in a 16mm 1.25" eyepiece for $44.95 . Poor Televue, they only could get 10mm eye relief with their 16mm Nagler.

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Well, a good thing I'm not considering Televues then!

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Garland Coulson
Orion XT8i Dob & Celestron 80 ED on a Vixen Porta Mount Mini
Baader Hyperion 8-24mm Click Zoom & Siebert Observatory 36mm
Siebert Black Knight Binoviewers
SkyWatcher Observing Chair
Celestron Skymaster 15x70mm binos

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The 1.6 OCA will allow you to go to 30mm easily without vignetting, which works out to about 18mm or so actual magnification.
The 16mm widefields work well in a BV, with no vignetting at all.
Actually, with a 1.6X oca in the Sieberts, you may even be able to go all the way to 35mm or even 40 without too much vignetting considering those in 1.25 size are all orthos or plossls.
Be wary of anything over 24mm that has a fov over 65 degrees, because that's where you may encounter some vignetting.

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My extensive experience with a Burgess binoviewer is that a) you certainly don't need wide angle eyepieces as they aren't as appreciated in a BV as in monovision and that b)high power greater than say, 150x, doesn't work as well in less expensive and more poorly collimated BV's than in the more expensive Denkmeier, TV, and Earthwin units. So, with a 1.6 corrector combined with the bino unit, your focal length will probably effectively exceed 2000mm (see Edz in a number of other threads on this point). Therefore, your 20mm Burgess binolites (which are wonderful in this application),will yield as you noted about 100x and 15mm eyepieces will probably provide as much additional magnification at 133x or so as you'll need or want. I have Hands-on-Optics GTO Plossls and they work great. Traditionally, the bad news about binoviewers is that they obviously require twice the number of eyepieces in a given f/l as monoviewing; the good news is, inexpensive eyepieces give you as much enjoyment in a binoviewer as ultra wide angle, well corrected TV eyepieces do in cyclops mode. At least, in my not so humble opinion. Tim

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The interesting thing about good binoviewers is that they make plossls and orthos look like Naglers and Pentaxes, while the Naglers and Pentaxes still look like Naglers and Pentaxes.
'tis a puzzlement.

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I've never tried it, but I have my doubts about using 2" eyepieces with a downshifting adapter (I don't even know who sells those), as it is almost sure to vignette very badly with the 22mm CA of the Siebert BV.

YMMV, but my experience using zooms (TV clickstops) was pretty unsatisfactory. I had a problem with matching up mags because the clickstop locations were not exact...a problem with most zooms.
I would take a bit of time and get familiar with using BVs and how things actually stack up before spending too much money ahead of time. The biggest variable is the individual's eye's and how they react to merging, and this and other eye anomolies can quickly change what you actually wind up needing, or wanting.

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I would skip buying extra eyepieces, and just use a standard barlow in front of the binoviewer. I have WO binoviewers and I have never bought any additional eyepieces, I use the included 1.6 mini barlow, then I have a Zhumell 2x barlow that the lens assembly actual unscrews from the front of the barlow and screws perfectly on the front of the binoviewer, it actually looks very much like the 1.6x corrector included with the binoviewer, I would guess it provides a around 2.5x screwed on the front of the binoviewer. Then I have a Celestron 2x barlow that I actually use in front of the binoviewer. I would guess it provides a bit over 3x in front of the binoviewer.

The advantage to doing things this way, is you are still using longer focal length eyepieces, which provide plenty of eye relief and also are less sensitive to collimation issues with the binoviewer. Also buying 2 cheaper barlow lenses is much cheaper than buying 4 eyepieces..

Good luck

Ken

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40mm eyepieces are not really a good match for a binoviewer. Almost without exception, the 40mm eyepiece field of view will be strongly vignetted. It starts out with a 40° Afov and gets vignetted to perhaps 35° Afov. A better selection of a finder eyepiece in a binoviewer would be a 28mm plossl which would show nearly the full 50° Afov, and almost exactly the same (if not slightly more) true field as the 40mm. A 30mm plossl will vignette but still show about the same Tfov as a 40mm.

The 2" eyepieces would be a poor choice. Any adapter to use them in a 1.25" scope would modify the fov to that of a 1.25" eyepiece, and you'd be dealing with all that extra weight that would surely cause your diagonaal some heartache.

Best eyepieces for a standard 1.25" binoviewer range between 12-14mm up to 25-28mm.

In actual drift timing tests using Burgess BV and Stellervue BV, both with and without OCS, when the OCS was in use for a refractor, a 26mm Meade 5K plossl (60°) and a 30mm Celestron Ultima (50°) both provided almost exactly the same Tfov (5 seconds time difference, the meade was 1.5% wider), and those were 9.5% and 8% wider than a 40mm TV plossl. The only real gain you get from eyepieces longer than about 26-28mm is lower power and a larger exit pupil, not a wider field of view.

Although I haven't tested it, I have used it, and I would expect the 32mm TV plossl might just give the widest achievable Tfov for a 1.25" eyepiece in a binoviewer. If it doesn't vignette any worse than these two, it's real Afov won't be any larger than that Meade 5K 26mm or that 30mm Ultima, but it will be operating at slightly lower power and may result in a wider Tfov.

For all eyepieces where the field stop is larger than the prism aperture in the binoviewer, the effective Afov is entrely dependant on the distance between the BV inside prism aperture and the field stop in the eyepiece. The closer they are, the more the fov is controlled by the diameter of the BV prism aperture rather than the eyepiece field stop. That's why the 40mm eyepieces do so poorly. The field stop in a 40mm eyepiece is almost right up at the front end of the chrome sleeve which puts it almost right on top of the BV inside prism aperture. Therefore the Tfov, instead of being a result of the 27.3mm field stop, is actually cut to 22.6mm, almost as small as the BV prism aperture.

edz

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