Binoculars for Skywatching

Binoculars are good for scanning wide areas of sky and viewing the moon. They are particularly good for your first "close-up" of the heavens. The image in binoculars is upright and visible with both eyes. Fairly good binoculars are less costly than the least expensive telescope of good quality. They are a good investment, since even if interest wanes, binoculars can be used for other purposes.

Get real binoculars, not "opera glasses." The latter are merly two simple Galilean telescopes side by side. They are smaller than binoculars and provide lower magnification, a smaller field of view, and an image of poorer quality. Some are made to look like binoculars; so - to be safe, buy from a reputable source.

True binoculars have a pair of prisms in each optical path to produce an upright image and make possible a long optical path within a short instrument length. Binoculars are available with magnifications from 3x to 20x or more, and with objective lenses from a few millimeters to several centimeters in diameter. They are usually classified by a pair of numbers such as "7x50", the first number indicating the magnifying power of the instrument, the "x" meaning "times", and the second number the diameter of each objective lens measured in millimeters. A 7x50 is a good all-around type for astronomical purposes. In general, lower-powered binoculars have wider fields; higher-powered ones are heavier and harder to hold steady by hand.

The higher the power with an objective of a given diameter, the more the light from the object is spread out and the less bright is the image. Often binoculars are rated in terms of Relative Light Efficiency, or RLE, easily calculated by dividing the diameter of the objective lens in millimeters by the square of the power. For a 7x50 instrument the RLE is 50/(7x), or about 1. A lower RLE means the iamge is less bright. An RLE of less than 1 is not recommended for astronomy, except for viewing the moon. An RLE of 1 will enable you to see objects about 20 times fainter than the unaided eye can detect.

For prolonged viewing, particularly with higher-powered binoculars, some support is needed. Various clamps are available for use with a camera tripod. This arrangement is an altazimuth mounting, but because the field is larger it is not a serious drawback. Some amateurs have devised equatorial binocular mounts.

Another useful accessory, available from suppliers, is a pair of small rubber cups to fit over the eyepieces of the binoculars to keep out extraneous light during viewing.

Never view the Sun through binoculars - they would act like burning glasses! Prolonged viewing of a full Moon also is inadvisable. On a clear, dark night 7x50 binoculars steadily held can reveal objects as faint as 9th magnitude, including star clusters such as the Beehive and Hyades, and even the planet Jupiter's four brightest satellites.

The picture on the right is the Comet Hyakutake. ----->

Telescopes

Telescopes are divided into three main classes: Refractors, which use lenses to gather light; reflectors, which use mirrors; and catadioptric systems, which use a combination of lens and mirror.

The main light-gathering component of any telescope is the objective. The greater its area, the greater the amount of light received. This increases as the square of the objective's diameter. The objective does not magnify; that task is for supplementary lenses. But adequate light grasp is needed to detect faint objects and render good image detail. A 3-inch lens gathers about 100 times as much light as the eye, which is why you must never view the Sun through a telescope without precautions.

An important tool in evaluating an objective is its f-ratio - the ratio of its focal length (F) to its diameter (D). Thus, where F = 48 inches and D = 6 inches, F/D = 8, the f-ratio, sometimes written f/8. An f-ratio of 3 to 6 is useful for wide fields of view and faint objects, but image size is small. For larger images, showing more detail on Moon or planets, an f-ratio of 10 to 20 or more is preferred. A telescope with an objective in the f/8 to f/10 range is practical for amateurs because it represents a compromise between wide field and high magnification.

To view an image formed by the objective, a lens called an ocular, or eyepiece, is placed near the focus. Eyepieces of different focal lengths provide different magnifications. Magnification (M) equals the focal length of an objective (F) divided by the focal length of the eyepiece (f). The use of magnifications less than about 3 or more than 50 times the diameter of the objective usually will give poor results.

In a reflector the objective is a mirror, called the primary. Because it reflects light (instead of transmitting it), another mirror, a small secondary, usually is introduced into the reflected beam to direct it to a better position for viewing. The amount of incoming light blocked by the smaller mirror is usually negligible. Two commonly used primary-secondary systems are shown; there are many others.

Catadioptric telescopes, such as the Schmidt and Maksutov systems, have a mirror with a spherical curve, so that a lens is needed to correct for aberrations. These telescopes typically have a wide field, small f-ratio, and longer focal length within a short space.

Resolving power is the ability of a telescope to distinguish between extremely close objects. The theoretical resolving power of an objective with D measured in inches is 4.54/D arc seconds; thus a good 6-inch telescope will "split" stars 0".76 apart.

To aim a telescope at a celestial object, a finder is used: a small telescope with a wide field, mounted on and aligned with the main instrument. With its wide field, the finder is easy to aim; and when it is aimed properly, so is the main instrument.