Binoculars (2)

In good-quality binoculars, all the optical surfaces, including the prisms, will be coated to improve light transmission and image brightness. This feature can make the difference between searching for a faint nebula and actually seeing it.

Before making a purchase, examine the binoculars carefully. Once you have decided on the appropriate magnification, compare prices of the same model in different stores. Make sure that you handle them and that the weight and bulk are within your limits of tolerance. Also ensure that the mechanical quality is up to standard; the central axle should be firm and the focusing mechanism smooth. The best way to test the optics is to look at a few distant objects. The image should be crisp all the way to the edges of the field of view. Watch also for colour rings around bright object.

Colour rings around the image indicate chromatic aberration, in which the component colours of the light source are not being brought into common focus. This fault is standard with a refracting instrument, including refracting telescopes, but it should not approach a noticeable level. To overcome the worst effects of chromatic aberration, multi-element lenses are used. These lenses are composed of a variety of glasses, each with a different refractive index, or ability to bend light.

Once purchased, all binoculars should be treated with care. Provided that the external glass surfaces are covered after use by the caps supplied, and that the binoculars are not subjected to any sharp knocks (which could loosen the lens elements) they should last a lifetime. If the lens surfaces are coated, avoid using any type of liquid lens-cleaning fluid; remove loose dust, using a blower brush.

Stereoscopic vision

Observing with both eyes through binoculars creates more of a sense of depth than through a telescope because a little of the sides of the object under scrutiny become visible. The widely spaced object lenses of binoculars reinforce this effect.

Coated glass surfaces

Better-quality optical instruments have all their internal and external air-to-glass surfaces coated with a substance designed to improve light transmission. This should eliminate nearly all glare and reflections and increase the brightness of subjects such as the Moon, above.

Choosing binoculars is not a matter of magnification alone, but also of the correct magnification for the object to be observed. The larger the magnification, the less the field of view of the instrument, as can be seen, above. The Moon as it could appear with the naked eye, 1 shows a complete disc. With 7 x 35 binoculars, 2 the disc is much larger, and since this size increases with 9 x 35 binoculars, 3 it can be seen that the field of view becomes much narrower. With a 20 x 50 instrument, 4 the complete disc is no longer visible.

Binoculars for use at night are known as night glasses and have larger objective lenses than their daytime equivalents. It is the object glass that collects the available light.

Optical alignment is important in binoculars. Unless both barrels and all the internal optics are perfectly parallel, the two images will not merge into one circle. Misaligned optics will have a detrimental affect on observable detail and can cause eye strain.

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