Telescope Reviews: Dual purpose binoculars

Let me warn you of one thing that you probably have considered and two that you may be completely unaware of. The first is what I think of as the "quality trap." I really liked your comment about not taking anything too expensive in a boat. I feel just the same about my equipment, and it pushes me away from owning the very best and most expensive optics. Once equipment gets expensive enough so that you hesitate to use it for fear of losing or damaging it, its utility is seriously impaired.

It's really easy to spend an extra factor of three or four for the difference between good optics and great optics. Is it worth it? That depends what you want. If you want to luxuriate in how wonderful your equipment is, or to admire how beautiful your view is, then maybe yes. If your overriding goal is to see alligators, or birds, or star clusters, then probably no. Yes, there is some practical benefit going from pretty-good optics to great optics, but the main difference is aesthetic, not practical.

On the other hand, don't aim too low, either. Beyond a certain level, poor optics really will begin to make the instruments less useable. Finding that golden mean isn't always easy.

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One thing you're probably not fully aware of is the importance of viewing angle. Most spotting scopes and almost all binoculars offer straight-through viewing -- you look into the eyepiece the same way the scope is pointed. That's the most natural view for terrestrial objects, and it also makes things much easier to find. But it's a serious problem for astronomical viewing -- at least for tripod-mounted instruments.

The obvious problem is that viewing things high in the sky with a straight-through instrument is literally a pain in the neck -- almost impossible for objects near the zenith. Less obvious is the extra demand it places on the tripod. Straight-through viewing requires the center of the instrument to be above your head, which requires an abnormally tall tripod. That, in turn, means that the tripod has to be extra heavy so that it doesn't wobble when extended that far.

For these reasons, almost all telescopes designed for astronomy allow you to view at right angles to the direction the scope is pointing. This is extremely desirable if you expect to use the scope much for astronomy.

A fair number of spotting scopes allow you to view at a 45-degree angle, which as far as astronomy is concerned is much better than straight-through but significantly worse than 90 degrees. Very few spotting scopes allow you to view at a 90-degree angle, or to vary the angle. Most astronomical telescopes -- refractors, anyway -- can be fitted with different "diagonals" to support 90-degree, 45-degree, or straight-through viewing.

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Finally, the optical demands are different for terrestrial and astronomical scopes. Aperture is very important for astronomy, but not nearly as important for terrestrial applications. If I were buying a small scope explicitly for astronomy, I'd be strongly drawn to 80-mm aperture. For most terrestrial purposes, 60 mm is fine.

Magnification is also much more important for astronomy. 100X is only modestly high for astronomy, but it's very rare to use that much magnification on terrestrial subjects. Consequently, scopes designed explicitly for astronomy tend to have higher optical quality than spotting scopes -- at least in the lower price bracket. You can get a 66-mm astro scope with superb optics for $400, but you'd probably have to pay $1000 or more for equivalent quality in a spotting scope. The extra money goes into things like ruggedizing and waterproofing which are much less relevant for astronomy.

For all these reasons, you really have to decide whether this scope is mainly for astronomy with a sidelight in terrestrial applications or vice versa. Both kinds of scopes work for both purposes, but the emphasis is quite different.

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Tony Flanders

eyeglasses
6x15 and 8x32 monoculars
8x25, 7x35, 10x30 IS, 10x50, and 15x70 binoculars
70mm and 100mm achromatic refractors
4.5", 7", and 12.5" Dobs