Telescope Reviews: Some tid-bits obout the 2.3x40 Owl eyes

Back in the early- to mid-nineties I purchased a pair, and still have them. Before that I'd never owned a Galilean type optic. (You know, the type which uses negative lenses for eyepieces and hence generally offer *very small* apparent fields of view.)

Well, with this little Russian beauty, the small field blues are negated by using the combination of short f/ratio objectives and strongly curved oculars. As with any Galilean instrument, the field of view is determined by the apparent angular diameter of the objective as seen through the eye lens. So the Owl eyes' wide objective allows a generous field such as I've never seen in any other "opera glass."

As marked on the instrument, the field is purported to be 28 degrees. As-is, I measured it closer to 18 degrees. For the longest time, therefore, I thought it was simply a typo. Experiments with positive/negative lens combinations revealed that the closer one's eye was to the eye lens, the larger the field of view. After fiddling around to see what could be done to enlarge the field, I found that the black "back plate" on the rear of the eyepiece, which has the hole one looks through, could be threaded off. With it removed, one's eye could get rather closer to the eye lens and hence enjoy a larger. NOW I get close to the advertised 28 degrees, and can just fit in the view the entire Big Dipper!

It's been so long now since I did this that I just can't remember... with the "back plate" removed, the eyepiece *may* be able to fall out. At any rate, whether to also keep it in place, I did wrap electrical tape around the exterior of the eyepiece housing mainly to keep the focus set.

An oddity I've discovered about my eyes is that I can relax them and to a significant degree focus "beyond infinity" (no Buzz Lightyear jokes, please!). It's like being presbyopic, or far-sighted. As a result, I tend to focus any instrument with the oculars moved farther back, which results in a slight increase in magnification. It just might be the case that for me, my Owl eyes work at closer to 2.5X (I should measure this, eh, Edz?). If so, this would correspond to a light gathering power of a factor of ~6, or nearly 2 magnitudes, assuming no significant light loss. Where I often observe from, my nelm is typically 6.4m. So the Owl Eyes should allow me to see down to ~8.3m.

If we use the naive calculation: Afov = Tfov x mag., we have Afov = 28 x 2.5 = 70 degrees. (Or perhaps closer to 64 degrees if the working mag. is indeed 2.3X.) Not bad, indeed! I should point out that because the "field stop" is the very out-of-focus image of the objective's edge, there is no nice and sharp black circle framing the view. But this is always the case for opera glasses.

About the optics themselves. I was quite surprised at their sophistication! The objective is an air-spaced two-element job. But I really can't recall if the ocular has one or two elements. All surfaces are coated, and have the typically Russian mauve/purple/tan color cast.

To get a symmetrically sharp image, centering of the eyes is rather critical, due mainly to the steeply curved oculars. But once achieved, the views are really quite nice! Due to the necessity of accurate eye centration, it's pretty much pointless trying to evaluate off-axis sharpness, as the test is effectively rendered invalid due to the now-incorrect pupil position. At any rate, when looking toward the field center, the peripheral sharpness as perceived by the off-axis retina is fully satisfactory in my opinion. And given that this is fundamentally an almost ancient design--albeit modernized--this is no mean feat considering the steeply curved elements.

One last point. The odd, ground surface at the back end of the eyepieces was an annoying source of scattered light. To get around this I used a very fine brush to cover it over with flat black paint. Highly recommended!!

I don't ever plan on parting with mine, unless something even better comes down the pike.

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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV

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