TFOV and 1.25"/2" eyepieces.

Hey guys,

For binoculars using interchangeable eyepieces, it would seem that one would want to obtain an eyepiece with the largest field-stop and AFOV possible to obtain the widest TFOV of the heavens. Is this assumption correct?

If so, it would appear that many of the 1.25" eyepieces on the market have field stops that do not exceed 30mm while 2" eyepieces have considerably larger field-stops but would seem to be out of the running due to thier physical size.

Any comments or experiences with eyepiece/binocular combinations?

Thanks,

Eric

Hi Eric,

You have pretty much summed up the tradeoffs, and touched on a frustration of mine. There are very few conventional binoculars that can take standard 1-1/4" eyepieces, let alone 2". The only ones that come to mind are the Vixen BT80 and one Oberwerk model.

The additional factor for TFOV is of course the focal length of the objective system. Shorter f.l.'s will produce wider true fields of view, but generally at the expense of chromatic aberration. I believe it would be hard to come up with much more than 3° TFOV in 80mm binos or 2.5° in 100mm binos using 1-1/4" eyepieces.

Some observers have reported being able to use 2" eyepieces, which, as you point out, open up the field stop significantly. You might try posing this question on the binoviewer forum, where all manner of standard eyepieces are used. It is possible that some 2" eyepieces might fit you, and others not.

The next problem would be actually finding binos that could take the 2" EP's. The only ones I am aware of are binoscopes made by Matsumoto or Astromeccanica using dual telescope OTA's.

Good luck,

Milt

One thing that I don't think has yet been mentioned in this thread is that paired 2" occulars have proved to be very troublesome for some close eyed individuals to merge images with. Mechanically, a 2" binoviewer/binocular poses some real challenges, and those challenges add to the cost geometrically.

Also, from a purely practical standpoint, I wonder if 100mm isn't about the upper limit for portable, reasonably durable, "conventional" binoculars. Beyond that, limitations of focal length and sheer mechanics make solutions very, very pricey.

Also, I agree with Kenny, that a very well done, very well corrected 70-75mm binocular that could take interchangeable eyepieces up to 60-75x should be more than sufficient for most of us. That's one reason that I got so excited about the new Miyauchi "Saturn" 22/40x71mm. Those don't quite address what I perceive as the best mag/apperature size combos. The next step up for them is 115x, which is probably overly amibitous. A really good set of eyepieces that yielded around 60x might be a better choice, but the best solution would be if they took standard 1.25" e.p.s and thus the user could determine the best mag/exit pupil etc. At f/8 they should be well enough color corrected, but at that focal length they would of course have a much narrower FOV than I would truly like. So then the question becomes do you want to go to a shorter focal length that has to be much better color corrected and thus has to cost lots more. Tradeoffs, tradeoffs, tradeoffs.

Mike Swaim