Our eyes can see only a few of the wonders of space, but we can see many more with the use of a telescope. Even a small telescope can help us to see distant stars and galaxies that are too faint to see with our naked eyes.
Space exploration is a great pathway by which scientists gain knowledge about the earth and answer many questions about the universe. Orbiting telescopes will allow scientists to observe distant galaxies which may provide information about the past. This scientific tool has the function of resolving celestial bodies and bringing new and more distant objects into view.
Students will be able to:
explain how a reflecting and refracting telescope works
compare and contrast a reflecting and refracting telescope
Perhaps the most obvious advantage of a telescope is that it magnifies far away objects, making them seem very close. However, large modern are built to gather a large quantity of light from faint objects. In addition to its ability to magnify an object and to gather light, a telescope also has resolving power, the ability to separate two close objects of moderate brightness. Viewed through a small telescope, the blurred images of two close stars may seem like a single bright object. Viewed through a larger telescope, however, the same bright object may appear to be two distinct and separate stars. Therefore the larger the telescope, the better its resolving power.
Elements of the Telescope
The telescope has either a lens or a mirror to collect the light from an object and form its image. This is accomplished by permitting the light to pass through the lens or to be reflected by the surface of the mirror. Where the light is brought to a focus and the image is formed is called the focal point. Its distance from the center of the lens or the surface of the mirror is the focal length.
The ratio of the focal length to the diameter of the objective is called the focal ratio (f-ratio). For example, a 10-inch, f-9 telescope has an objective with a diameter of 10 inches and a focal length of 90 inches. The f-ratio for refracting telescopes is about 15 and for reflecting telescopes is about 6. The image formed at the focal point is examined with an eyepiece which is simply a magnifying glass of short focal length.
The image that is formed by a telescope has three important properties: size, brightness and resolution. The size of the image depends on the focal length of the objective. Therefore the size of the image increases as the focal length increases. A 10-inch f-5 telescope whose focal length is 50 inches forms a smaller image than a 10-inch, f-8 telescope whose focal length is 80 inches.
The amount of light collected by a telescope is proportional to the square of the diameter of the objective. The light-gathering power of a 2-inch telescope is 4. When the diameter of the objective is doubled, the light-gathering power is increased four times. The brightness of a star's image depends upon the amount of light collected by the objective. As the size of the objective is increased, the image becomes brighter, and more of the dimmer stars become visible. Two telescopes with objectives of the same diameter collect the same amount of light, regardless of their focal lengths.
A telescope does not reproduce the geometrical shape of a star; it simply produces an image called a diffraction pattern. This image consists of a bright central spot containing about 85% of the total light surrounded by faint concentric rings. The size of the star's image depends on the diameter of the objective and actors the wavelength of the light and has no relation to the brightness of the star.
The two general types of optical telescopes in use today are refractors and reflectors. In both types, the objective which produces the image is a fixed part of the telescope. In a refractor telescope, the objective is a lens and in a reflector telescope, the objective is a mirror. The eye-piece which is a magnifying lens of short focal length is easily interchangeable. The essential features of a refracting telescope are two lenses separated by a distance equal to the sum of their focal lengths. The larger lens serves as the objective and the smaller lens as the eyepiece of the telescope. Light from an object is collected by the fixed objective which it receives in parallel rays, focuses it and forms its image at the focal point. The moveable eyepiece then receives the light from the focal point, magnifies it and transmits it to the eye in parallel rays.
In a reflecting telescope, the objective is a mirror, which reflects the parallel rays of light it collects from an object and focuses them at the focal point where the image is formed. See diagram below.