Probably the best views of celestial objects will be obtained using prismatic binoculars (Figure 3.1). In this design, light passes through the objective lenses and is reflected by prisms before being focused at the eyepieces. Prisms extend the effective focal length of binoculars without increasing their size and create a sharper image than would otherwise be produced. This is especially important when observing double stars; the components should appear as individual pinpoints of light. They also invert the image, resulting in an upright view.
Image stabilised binoculars include advanced design features such as microprocessor controlled variableangle prisms. These compensate for involuntary move-
ment, enabling the observer to "lock on" to a celestial object at the press of a button. The increased steadiness of the image allows a higher magnification to be used without a tripod or dedicated mount. Comparisons with conventional binoculars have been most impressive. (For a list of test double stars see Chapter 2.)
Another feature of good-quality binoculars is coated or bloomed lenses, where the optical surfaces are treated with a substance to reduce the amount of light reflected from them. The resulting field of view is brighter and free from halos and other false images. Bloomed lenses appear blue or purple when studied under white light - a helpful point to remember if the binocular housing has not been stamped with the words "Coated Optics".
"Optional extras" could include eye-cups, which are circular pieces of plastic or rubber fitted around the eyepieces. Eye-cups prevent stray light from entering the eye and are particularly useful when observing from brightly-lit surroundings.
Most binoculars achieve focusing by means of a manually rotated centre-wheel located on the axis joining the optical systems. Additionally, it is common for one eyepiece to be individually adjustable ensuring that each image is correctly focused for the observer's eyes.
Finally, lens caps. Binocular lenses are delicate items and may incur damage by accidental scratching. A set of tightly fitting covers for eyepieces and objectives will provide valuable protection from mishap and ensure optimum performance is obtained for the lifetime of the binoculars.
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