The amateur astronomer

Many of the world’s astronomers are amateurs—in the sense that they do not earn their livings from pursuing the study of the science. This does not imply, however, that all such amateurs pursue their interest with unprofessional skills and equipment. Many indeed have made contributions of outstanding importance to astronomy, and some have equipment that some smaller observatories might envy. The amateur’s greatest asset is the opportunity simply to observe the heavens and investigate any object or phenomenon that seems of interest. In contrast, professional astronomers must follow specific programs of scientific research.
Amateur astronomers may be considered to fall into three approximate categories.
The first group includes those who have a general interest in the stars and planets. They can identify the more obvious constellations and follow the special astronomical events that are covered by the press or television. Members of this group are mainly naked-eye observers, although they may also use binoculars or small, tripod-mounted telescopes.
Binoculars (typically 7 by 50 or 10 by 50) with 7- or 10-fold magnification and an objective lens of 2 inches (51 millimeters) in diameter, will reveal many more stars than are visible to the naked eye and will give breathtaking views of the Milky Way. Craters and other surface features of the moon, Jupiter’s four main satellites, star clusters, and comets can also be observed.
Small telescopes, with objective lenses of a 2-inch diameter or more, are the next step from binoculars. Such small telescopes, which are capable of magnifications of up to eight times per inch of objective lens aperture, are not intended to be hand-held. They are usually provided with a light tripod, which provides some stability, and with an alt azimuth mounting, which is simple to use. This type of stand allows the telescope to be rotated about vertical and horizontal axes and appeals to those who may be mystified by sidereal time and setting circles or who simply want to point their telescopes at some interesting object in the sky.
A large number of books, maps, and star atlases are available to help locate objects in the night sky and can be used with alt-AZ instruments. Like many of the star charts that are issued monthly, these give altitudes, azimuths, and indicate the times when objects of interest can be observed.

Amateur astronomer observing a nebula in the constellation of Orion. It is important to allow the eyes to adjust to the dark before looking at the night sky. Most people’s eyes need from 10 to 30 minutes in darkness in order to adjust

The second group make amateur astronomy their major interest, either buying a relatively powerful telescope or even making a telescope for themselves. Telescopes they are likely to use are of three main types: refractors, reflectors, and catadioptrics, the last combining elements of each of the first two. Amateurs in this group would normally use a refractor with an objective diameter between 3 and 6 inches (75 and 150 millimeters), or a reflector with an objective diameter between 6 and 20 inches (150 and 510 millimeters). Catadioptrics are available with objective diameters of 3 and 14 inches (75 and 355 millimeters), the most popular sizes being 6 inches (150 millimeters) and 8 inches (200 millimeters).
Telescopes used by the second group are almost invariably equatorially mounted, which means that one axis points to the north or south celestial pole in the Northern or Southern Hemisphere, respectively. (This polar axis is parallel to the axis of the earth.) The other axis, at right angles to the polar axis, is the equatorial axis.
Two setting circles are important accessories for the proper use of an equatorial telescope: the right ascension circle is calibrated in hours and minutes of Right Ascension (R.A.); the declination circle is calibrated in degrees. A drive mechanism, which is usually electronic, regulated by a sidereal clock, turns the telescope about its equatorial axis so that, when set for the R.A. and declination of a particular star, the telescope is able to follow that star’s movement through the sky.
The sidereal drive actually turns the telescope at the same rate as the earth, but in the opposite direction. This capability is useful enough when observing through the eyepiece, but it is indispensable when photographing the moon, or planets, stars, nebulae, or other celestial phenomena, because long exposures are required.
Astrophotography is itself a rewarding branch of amateur astronomy for which neither exacting skills nor heavy expenditure is required. A single lens reflex camera can be used by attaching the body of the camera (without its lens) to a telescope from which the eyepiece has been removed. This way the primary image of the telescope objective is focused on the camera film. The telescope without its eyepiece simply becomes a powerful telephoto lens. This arrangement, using a telescope with a focal length of about 47 inches (1.2 meters), will give an image of the moon 0.4 inches (10.2 millimeters) in diameter at the focal plane of the camera and will look reasonably clear on 35mm film. Images can be increased in size by using eyepiece projection. Camera adaptors for this purpose can be obtained from most telescope suppliers.
The third group includes the most serious and advanced amateurs of all. They may actually be attached to university departments or observatories and are likely to be well qualified in related branches of physics, mathematics, optics, or electronics.

Particularly interesting subjects for amateur astronomers include:
The sun—especially: sun-spots; eclipses; also spec-troheliography (for which some amateurs can construct their own equipment). Because it will cause instant, permanent blindness, it is extremely important to remember to never look at the sun either directly, through darkened glasses, or through binoculars or a telescope. Observations can only be made by projecting the sun’s image onto a screen.
The moon—especially: surface changes; observations of the exact times at which the moon occults stars and planets; and photography, for which the moon is an ideal subject
The planets—especially: the rings of Saturn; the phases of Venus; the ice caps and markings of Mars; the movements of the satellites of Jupiter.
Comets, meteor showers, and aurora—amateurs make a particularly important contribution in this area because of the time they can spend simply searching the sky. Most comets have been discovered by amateurs.
Double stars or binaries—it has been estimated that roughly half the stars in our galaxy are binaries, showing variable brightness as the two stars orbit and eclipse each other.
Other deep sky objects—for example, nebulae; galaxies; star clusters; variable stars.