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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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My plan was to go for a pair of 1.25" 40mm plossls with 52 degree FOV, not 2" 40mms with a wide field of view.

I will wait for the 2" wide ones for if I upgrade to a 2" binoviewer. I might do this if I fall in love with binoviewing.

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Garland Coulson
Orion XT8i Dob
Baader Hyperion 8-24mm Click Zoom & Siebert Observatory 36mm
SkyWatcher Observing Chair
Webmaster, Nanaimo Astronomy Society

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