Optical telescopes, which form images of faint and distant stars, can collect much more light from space than the human eye can. Optical telescopes are built in two basic designs.
The heart of any telescope is its objective. The objective is a lens (in refractors) or mirror(in reflectors) whose function is to gather light from a sky object and form its image. The amount of light collected by the objective is called its light gathering power. Light gathering power is proportional to the area, or to the square of the diameter, of the collector. The size of a telescope, such as “6-inch” or “200-inch” refers to the diameter of its objective.
You can look at the image formed by the objective through and eyepiece, a variety of other ways. Your eye lens size is about 1/5 inch. A 6-inch telescope has an objective that is thirty times bigger than your eye lens. Its light gathering power is (30)2, or nine hundred times greater than your eye’s. So a star appears nine hundred times brighter with a 6-inch telescope than it does to your naked eye.
All stars appear brighter with telescopes than they do to your eye alone. All the extra starlight gathered by the telescope is concentrated into a single point. Using time exposure, the 200-inch telescope can photograph very faint stars down to nearly magnitude 24 (see frame 1.7). A star of that magnitude has about the same apparent brightness as a candle viewed from 10,000 miles away.
The Refracting telescope
The refracting telescope has a large lens (the objective) permanently mounted at the front end of the tube. Starlight enters this lens and is refracted, or bent so that it forms an image near the back of the tube.
The distance from this lens to the image is its focal length. You look at the image through a removable magnifying lens called ocular, or eyepiece. The tube keeps out scattered light, dust, and moisture.
Galileo Galilei first pointed refracting telescopes skyward in Italy in the seventeenth century. The largest instrument he made was smaller than 2 inches. Today refracting telescopes range in size from a beginner’s 2.4-inch (60-mm) version to the world’s largest, the 40-inch (12-cm) telescope at the Yerkes Observatory in Williams Bay, Wisconsin.
The reflecting telescope
The reflecting telescope has a highly polished curved glass mirror (the objective) mounted at the bottom of an open tube. When starlight shines on this mirror, it is reflected back up the tube to form an image at the prime focus.
You can place the photographic film at the prime focus to record the image, or you can use additional mirrors to reflect the light to another spot for viewing. The Newtonian reflector uses a small flat mirror to reflect the light through the side of the tube to an eyepiece. The cassegrain reflector uses a small convex mirror to reflect the light through a hole cut in the objective mirror at bottom of the tube.
Reflecting telescope range in size from a beginner’s 3-inch Newtonian reflector to the world’s largest the 236-inch (6-m) reflector in Caucasus mountains in the soviet union. The largest reflectors in the United States is the 200-inch (508-cm) Hate Telescope on Mount Palomar in California.
Telescopes are often described by both their objective size and f-number. The f-number is the ratio of the objective to its diameter. These specifications are important because the brightness, size, and clarity of the image produced by a telescope depend on this diameter and focal length of the objective. For example,”6- inch, f/8 reflector “ means the objective mirror is 6 inches in diameter and has a focal length of 48 inches (8 times 6).
Image size is proportional to the focal length of the telescope’s objective. For example, a mirror whose focal length is 100 inches produces an image of the moon that measure almost 1 inch across. You know that the 200-inch mirror has a focal length of 660 inches, which is over times longer. Hence, it produces an image of the moon that is about six times bigger, or about 6 inches across.
Lenses and mirrors and from real images that are upside down. (A real image is formed by the actual convergence of light rays.) All stars except our sun are so far away that they appear as dots of light. The moon and planets appear as small disks. Since inverted images do not matter in astronomical work, nothing is done to turn them upright in telescopes.
Even if a telescope were of perfect optical quality, it would not produce perfectly focused image because of nature of light itself. A telescope’s resolving power is its ability to produce sharp, detailed image under ideal observing conditions. Resolving power is proportional to the diameter of the objective.
Starlight travels are straight lines through empty space. But when waves of starlight pass close to the edge of a lens or mirror, they spread out, in an effect called diffraction, and com to a focus at different spots. Because of diffraction, the image of a star formed by a lens or mirror appears as a tiny blurred disk surrounded by faint rings, called a diffraction pattern, instead of as a single point of light.
If two stars are close together, their diffraction patterns may overlap so that they look like a single star. Features such as moon craters and planet marking are also blurred by diffraction.
Resolving power indicates the smallest angle between two stars for which separate, recognizable images are produced. The resolving power of the human eye is about one minute of are (1’).
A telescope magnifying power is the ratio of the apparent size of an objective seen through the telescope compared its size seen by the naked eye. Telescopes magnify the angular diameter of objectives. Thus the images appear to closer than the object.
For example, to your naked eye the apparent size of the full moon is ½0, the same as an aspirin held at arm’s length. If the apparent size of the moon increases twenty times (so that it looks 100 in diameter When you view it through your telescope, then the magnifying power is 20, written 20X.