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One of the most wonderful experiences in amateur astronomy does not involve a telescope: scanning dark skies with a pair of binoculars. Cruising along the Milky Way with a pair of 7 X 50s or 10 X 50s, going from glittering star cluster to wispy nebula, it is hard not to think this is the way deep sky observing was meant to be.

What makes binocular observing so pleasurable? One thing is a binocular's wide fields, but there is more to it than that. The main reason binocular observing is so much fun? You are relaxed and using both eyes as nature intended, instead of squinting through one eye. As has been said many times, the more relaxed you are, the more you will see. If only it were possible to look through a CAT with two eyes.

Well, it is, with the aid of a device called a binoviewer (Plate 45). A binoviewer is similar to the binocular heads used on some microscopes. Light that enters the unit from the telescope is split into two paths by prisms and is sent to each of two eyepieces. Observers who have not used a binoviewer sometimes question whether this is a practical arrangement. Does not running the light through prisms and

Plate 45. (Binoviewer) Denk-meier's binoviewer allows observers to use both eyes while viewing, just as nature intended. Credit: Author.

splitting it into two dim images mean that little can be seen at the eyepiece? Images are dimmer in binoviewers than in a single-eyepiece setup, but in the best models the loss is small. This light reduction does not do any harm in viewing the solar system, and its effect on deep sky objects is surprisingly small in good binoviewers.

The ground truth is that you can see more detail more easily in any object using a binoviewer than you can with a single eyepiece. The object may be dimmer, but you still get a better view of it. A good example is the faint nebula just north of M42 in Orion, the Running Man, NGC 1973. I had wanted a good look at this combination of reflection nebula and dark nebula for a long time but never could get one, not even in fairly large Dobsonians. One night under a dark Chiefland Astronomy Village sky, just after receiving my first binoviewer, I gave NGC 1973 another try. I sent my go-to Nexstar 11 to its location, focused up, and there it was. It was not even a difficult observation. I removed the binoviewer to be sure it, and not exceptional skies, was what was making the difference. Sure enough, when I viewed with a single eyepiece—"Cyclops style" as the binoviewer fans call it—the Running Man disappeared.

All this does not mean that a binoviewer is necessarily for everybody. One stumbling block for many people is the fact that two "copies" of every eyepiece are needed for the binoviewer. For best results, these must not only be the same focal length, brand, and design, but also they should have been manufactured at close to the same time. Eyepiece makers incorporate small changes in eyepieces from time to time, which can cause problems for binoviewers. What kind of problems? These mainly involve merging images.

If there is something wrong with a binoviewer, the eyepieces, or the observer's eyes, it may be impossible to successfully merge two images into one. Instead of seeing one Jupiter, there will be two. That is not only unattractive, it often leads to a serious headache. Even with identical eyepieces and a binoviewer that is in perfect optical alignment, some observers have considerable difficulty with merging. Try to use a binoviewer at a star party before investing a lot of money in one.

How much will a binoviewer cost? Like everything else in astronomy, they have gotten cheaper recently due to those ubiquitous Chinese optical factories. Orion, for example, sells one for $170. These bargain units may not be a good investment, however. Binoviewers are not something to skimp on. Beyond the question of mechanical quality—a binoviewer must have its optics perfectly aligned, and these optics must stay perfectly aligned—there is the question of clear aperture. The small prisms in inexpensive units mean longer focal length eyepieces will vignette; their fields will be cut off. That reduces these binoviewers' usefulness for observing wide, deep sky vistas. That said, for the money, the Orion and similar units do a good job.

Be prepared to invest enough money for a setup that will allow the full binoview-ing experience: merged, bright images of solar system and deep sky objects. Two high-quality units are the TeleVue BinoVue ($840) and a personal favorite, the Denkmeier Optical Denk II ($900). Denkmeier also makes a basic binoviewer, the Standard, that performs almost as well as the Denk II at the more manageable price of $350. The TeleVue and the Denkmeier are of excellent mechanical quality and use top-quality optics with coatings that maximize light throughput.

At this point, you are loaded down with accessories, but now comes the payoff: Get out under the stars and start using that beautiful new CAT and all that cool astrostuff.


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