Refracting Telescopes Help (page 2)
The first telescopes were developed in the 1600s. They employed lenses. Any telescope that enlarges distant images with lenses alone is called a refracting telescope .
Galileo Galilei, the astronomer who became famous during the 1600s for noticing craters on the Moon and natural satellites orbiting Jupiter, devised a telescope consisting of a convex-lens objective and a concave-lens eyepiece . His first telescope magnified the apparent diameters of distant objects by a factor of only a few times. Some of his later telescopes magnified up to 30 times. The galilean refractor (Fig. 19-10a) produces an erect image , that is, a right-side-up view of things. In addition to appearing right-side up, images are also true in the left-to-right sense. The magnification factor , defined as the number of times the angular diameters of distant objects are increased, depends on the focal length of the objective, as well as on the distance between the objective and the eyepiece.
Fig. 19-10 . The galilean refractor (a) uses a convex objective and a concave eyepiece. The keplerian refractor (b) has a convex objective and a convex eyepiece.
Galilean refractors are available today mainly as novelties for terrestrial viewing. Galileo’s original refractors had objective lenses only 2 or 3 cm (about 1 in) across; the same is true of most galilean telescopes today. Some of these telescopes have sliding concentric tubes that provide variable magnification. When the inner tube is pushed all the way into the outer one, the magnification factor is the lowest; when the inner tube is pulled all the way out, the magnification is highest. The image remains fairly clear over the entire magnification-adjustment range. These instruments are sometimes called spyglasses .
Johannes Kepler, whose audience was more friendly than Galileo’s when it came to his theories concerning the universe, refined Galileo’s telescope design. Kepler’s refracting telescope employed a convex-lens objective with a long focal length and a smaller convex-lens eyepiece with a short focal length. Unlike the galilean telescope, the keplerian refractor (see Fig. 19-10b) produces an inverted image ; it is upside-down and backwards. The distance between the objective and the eyepiece must be exactly equal to the sum of the focal lengths of the two lenses in order for the image to be clear. The magnification factor depends on the ratio of the focal length of the objective to the focal length of the eyepiece.
The keplerian telescope is preferred over the galilean type primarily because Kepler’s design provides a larger apparent field of view . Galilean telescopes in general have apparent fields of view so narrow that looking through them is an uncomfortable experience. The magnification factor of a keplerian telescope can be changed by using eyepieces with longer or shorter focal lengths. The shorter the focal length of the eyepiece, the greater is the magnification factor, informally known as power , assuming that the focal length of the objective lens remains constant.
The largest refracting telescope in the world is located at the Yerkes Observatory in Wisconsin. Its objective lens has a diameter of 40 in (slightly more than 1 m). Keplerian refractors are used by thousands of amateur astronomers worldwide.
Limitations Of Refractors
There are certain problems inherent to telescopes that use objective lenses. These are known as spherical aberration, chromatic aberration , and lens sag .
Spherical aberration results from the fact that spherical convex lenses don’t bring parallel light rays to a perfect focus. A refracting telescope with a spherical objective focuses a ray passing through its edge a little differently than a ray passing closer to the center. The actual focus of the objective is not a point but a very short line along the lens axis. This effect causes slight blurring of images of objects that have relatively large angular diameters, such as nebulae and galaxies. The problem can be corrected by grinding the objective lens so that it has a paraboloidal rather than a spherical surface.
Chromatic aberration occurs because the glass in a simple lens refracts the shortest wavelengths of light slightly more than the longest wavelengths. The focal length of any given convex lens is shorter for violet light than for blue light, shorter for blue than for yellow, and shorter for yellow than for red. This produces rainbow-colored halos around star images and along sharply defined edges of objects with large angular diameters. Chromatic aberration can be almost, but not completely, corrected by the use of compound lenses . These lenses have two or more sections made of different types of glass; the sections are glued together with a special transparent adhesive. Such objectives are called achromatic lenses and are standard issue on refracting telescopes these days.
Lens sag occurs in the largest refracting telescopes. When an objective is made larger than approximately 1 m in diameter, it becomes so massive that its own weight distorts its shape. Glass is not perfectly rigid, as you have noticed if you have seen the reflection of the landscape in a large window on a windy day. There is no way to get rid of this problem except to take the telescope out of Earth’s gravitational field.
Practice problems of these concepts can be found at: Optics Practice Test
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