Whether you are sweeping the Milky Way, scanning star fields or observing the passage of a bright comet, a good pair of binoculars will present a magnificent, wide-field view of the night sky. And because both eyes are being used, the images you observe have a real appearance of depth. Observing in this way is a great deal more restful than using a telescope, and thus longer observation periods are possible.
Significant astronomical discoveries have been made by knowledgeable amateur observers using very powerful binoculars; for example, many comets have been found. Variable star observers also find binoculars useful, since many variables, are well within range.
Binoculars with magnifications of more than 7 times should be mounted.
The reason for this is that the more powerful types are heavy and difficult
to hold steady, and even the pulsing of one's blood can cause the binoculars
to waver. The most common method of mounting binoculars is to use a camera
tripod. Adaptors are available that clamp around the central axle of the
binocular and attach to the tripod camera mounting bolt.
A
tripod (left) should be used to mount any binoculars with a magnification
of more than about 7 times. Some large instruments may have a threaded
hole designed to accept the retaining screw of a standard photographic
tripod. If not, adaptors are available that clamp on to the central axle
of the binoculars.
These are two types of binocular roof prism and Porro prism. The roof prism type is usually smaller and easier to handle because its directlight-path design allows a more compact construction. Most astronomers, however, are more familiar with the Porro prism variety. These instruments have the typical binocular shape but contain bulkier prisms. These are present in order to orientate correctly the image (which would otherwise appear inverted). Porro prism binoculars are thus suitable for both terrestrial and celestrial use.
Porro
prism binoculars (left) use two pairs of prisms to produce an image that
corresponds to normal vision. As a result, they are suitable for both celestial
and terrestrial use, but are bulky and can, be awkward to hold for long
periods of observation.
Roof
prisrn binoculars are optically simpler than Porro prism types. The single
prisms produce an inverted image, making them suitable only for celestial
use. They are relatively light in weight and suitable for long periods
of observation.
Binocular types
Binoculars are classed according to magnification and aperture. A size of 7 x 50, the most suitable size for general astronomical work, indicates a magnifying power of 7 times and an object glass diameter of 50 mm. The exit pupil is the bright disc of light emitted by the eyepiece, a measurement obtained by dividing the aperture by the magnification. For best results, the size of the exit pupil should match that of a fully dilated human pupil (about 7 mm).
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