Binoculars can be used for everyday observing, such as sporting events and nature study, as well as astronomy. They are classified according to their magnification and the diameter of the objective. A 10 X 50 pair magnifies 10 times (100 areas) and has objective lenses 50mm in diameter; note, however, that in cheaper binoculars the objective may be stopped down internally in order to improve the sharpness of the image. This reduces the light-gathering power of the binoculars. Remember that larger magnifications give smaller fields of view. Internal reflections on the optical surfaces reduce the amount of light passing through binoculars. Better-quality instruments have bloomed (or coated) lenses, which help to reduce the light-loss. Some cheap instruments may have the outer surface of the objective ostentatiously coated, but this does not imply that all the lenses are bloomed.
Magnification between 7 and 10are most suited to celestial work, At higher magnifications it becomes very hard to observe because the unsteadiness of one’s hands is magnified as well! For this reason the maximum aperture that is suitable is about 50mm, because anything larger will be too heavy to hold steadily. Lenses smaller than 30mm do not gather enough light for worthwhile astronomy. This brackets the range you should consider from 7 x 30 to 10 x 50. Of course, if you want to take up comet spotting, or other specialized work, larger binoculars will be needed, perhaps up to 20 x SO, requiring special mounts for effective use. Our concern here is to get you started for the minimum expenditure.
When choosing binoculars there are a few points to remember. Blooming of the lenses will show up as a blue-purple or yellow colour. Establish, if you can, whether the prisms are held in by adhesive. Prisms mounted in this way tend to come out of alignment relatively easily. A reputable dealer will be able to advise on this matter. Look down through the objectives for chips or blenches in the lenses or prisms: if there are any defects at all, do not buy. See that all the moving parts function correctly, and assess whether they are robust, The next test is to hold the instrument in outstretched arms and look at the disc of light (exit pupil) visible in each eye-piece. It should be perfectly round and of even brightness. If the instrument passes this test look through it and check for distortion and aberration. In most countries of the world acceptable binoculars will cost you about 1.5 to 4 times the current cost of this book.
Having found suitable binoculars, what phenomena can be ob¬served ? First of all, it must be clearly understood that the Sun must not be viewed through binoculars under any circumstances what¬ever. Even when low on the horizon, and therefore deceptively harmless, it is a potential source of danger. The Moon is a splendid sight. The appearance of its surface changes through the lunar month as the angle of solar illumination alters. At full Moon, brilliant rays may be seen emerging from the brightest craters, although the craters themselves do not stand out sharply at this time, since the Sun is shining on them from overhead. At other phases (crescent, hall three-quarters) the mountains, valleys, and craters near the terminator – that is to say the boundary between the dark and bright sectors of the Moon – show up magnificently. When the Moon is at narrow-crescent phase, it is also possible to see some features on the dark part of the Moon, which is illuminated by reflected light (Earthshine) from Earth.
For planetary observations, binoculars will give a glimpse of the crescent phase of Venus. Mars and Saturn will show no details, although it is often evident that Saturn is more than a simple disc. Jupiter is the most rewarding object, since its four major satellites present an ever-changing aspect from month to month. They look like a set of stars, strung out in a line, in the neighbourhood of Jupiter.
When binoculars are turned onto the stars they will not, of course, resolve any of them into discs. But many observers claim that the colours are more evident for the brighter stars. Some binary pairs can be separated, such as Mizar in Ursa Major, Vega in Lyrae, and, in southern skies, ? Tucanae, a superb binary consisting of two 4.5 magnitude stars. Binoculars can be used to great advantage to sweep star fields, because they have a wider field of view (typically 7°-9°) than a telescope.
The Milky Way is staggeringly beautiful, and is resolved into the myriads of stars first glimpsed by Galileo; the richest star fields are in Sagittarius. Star clusters are also impressive under low power. Northern observers can see dozens of stars in the Pleiades, the Hyades (in Taurus) and Praesepe (Cancer). In the southern hemisphere, M23 (Sagittarius). M6 and M7 (Scorpius) will show a great number of stars. Besides these open clusters, some globular clusters can be found. In this case, southern skies have a decided advantage, being supplied with 47 Tucanae and co Centauri, two globular clusters making a noble sight under low power.
Gaseous nebulae can be viewed with binoculars. Examples to try are the Orion nebula and 30 Doradus in the Large Magellanic Cloud. The spiral galaxy in Andromeda, M31, is an easy object that will show a soft oval outline, while the Magellanic Clouds are simple extragalactic targets in the south.
From these examples you can see that much can be achieved with good binoculars. A keen observer may consider graduating to a telescope proper. Here the choice is much wider and will be restricted mainly by your personal budget. However, a few general remarks will help you choose the telescope best suited to your interests.