Hand-held binoculars are the choice for extreme portability, casual observing, and as a preliminary "sky-scanner" used in conjunction with a larger instrument. Almost all binoculars, including cheap plastic opera glasses, will show you more than the naked eye, but a sensible lower limit of aperture for portable astronomical binoculars is 30 mm. If extreme portability is not an issue, 40 mm is significantly better as it will admit more than 75 percent more light. As aperture increases, so does the weight of the binocular, making it increasingly difficult to hold steadily. The sensible upper limit of aperture for hand-holding is normally considered to be 50 mm. Larger apertures than this can be held for short periods, but are too tiring to use for anything other than very brief views.
For many years the commonsense view was that the limit to magnification for hand-held 50-mm binoculars was x7. This is probably because 7x50 was, and still is, the most common size of hand-held marine binoculars. While it is true that they are easier to hold steady than, say 10x50, most of us do not do our astronomy from the moving deck of a boat. There are very few astronomical objects that are better at 7x50 than at 10x50, and it is perfectly possible to hold 10x50 binoculars sufficiently steady when our observing platform is the ground (see Chapter 6).The increased magnification of the 10x50 allows us to see more detail and generally gives more satisfying views. This is reflected in its higher rating in every performance index except relative brightness, and even this difference is reduced for those of us whose pupils do not dilate sufficiently to enable us to use the full 7-mm exit pupil of the 7x50 (Figure 3.2).
There is perennial debate on whether, if you use small or medium-sized binoculars for astronomy, you should use Porro prism or roof prism binoculars. The
Figure 3.2. Variety of 10x50 porro-prism binoculars. L to R: Older style center-focus Z-body with fixed eyecups (Zenith); Robust center-focus B-body style (Swift Newport); Modern lightweight center-focus Z-body with folding eyecups (Helios Naturesport); Robust individual focus Z-body (Strathspey Marine).
conventional wisdom is that Porro prism binoculars are better for astronomy. While it is certainly true that for the same price Porro prism binoculars tend to have superior optical quality, good quality roof prism binoculars are as optically and mechanically good as good quality Porro prism binoculars. The roof prism binoculars tend to be lighter and more compact and are therefore generally easier and more comfortable to hold. Over recent years I have found that I use my 10x42 roof prisms more than my 10x50 Porro prisms, and a side-by-side comparison shows that I see no more in the Porros when I hand-hold them, although they do show very slightly more when they are mounted. Roof prism binoculars also have the advantage that they can more easily be made waterproof, as the focusing mechanism is usually internal.
If you choose Porro prism binoculars, you may also have a choice between center-focus and independent eyepiece focus. There are no advantages to center focus if the binoculars are to be used exclusively for astronomy, but if you intend to use them for terrestrial purposes (e.g., bird watching or horse racing) then you should get center focus. My preference for astronomy is independent focusing eyepieces. They tend to be more mechanically robust, do not suffer from a rocking bridge, and modern ones tend to be waterproof and nitrogen filled, reducing the likelihood of internal fogging.
Another option for hand-held binoculars is image stabilization. This feature incorporates an electronic system that compensates for motion and vibration. Different manufacturers employ different stabilization systems, which were developed initially for military surveillance and not for astronomy. A test report in Sky & Telescope suggests that the best stabilization system for astronomical observation is that employed by Canon, whose optics were also superior.4 In addition to the stabilization system, the optics are essentially a roof prism system with a field flattener (see Chapter 2) incorporated into the design. The stabilization system (see Chapter 2) compensates for shake and the result is that you can see fainter objects and more detail. A 10x30 IS binocular will show most people more than a conventional 10x50. While a 10x30 IS is sufficiently lightweight (600g/1.25lb) to be held for relatively long periods, the larger 15x50 IS and 18x50 IS are heavy enough to be tiring
to hold for extended periods. If you are considering purchasing image stabilized binoculars, you should be aware that the image stabilization mechanism requires battery power, and that the life of batteries, particularly alkaline batteries, is reduced in cold weather. Without power, the binoculars can be used as conventional binoculars (Figure 3.3).
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