Reflection and Refracting Telescopes Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 18, 2011

Limitations Of Refractors

A well-designed refracting telescope is a pleasure to use. Nevertheless, there are certain problems inherent in their design. 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. Thus a refracting telescope with a spherical objective will focus 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 paraboloidal rather than spherical surfaces.

Chromatic aberration occurs because the glass in an objective lens refracts the shortest wavelengths of light slightly more than the longest wavelengths. The focal length of any given convex lens therefore is shorter for violet light than for blue light, shorter for blue light than for yellow light, and shorter for yellow light than for red light. This produces rainbow-colored halos around star images, as well as along sharply defined edges in objects with large angular diameters. Chromatic aberration can be 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 in refracting telescopes these days.

Lens sag occurs in the largest refracting telescopes. When an objective is larger than approximately 1 m (about 40 in) across, 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 with a refractor except to take the telescope out of Earth’s gravitational field.

Reflecting Telescopes—Newtonian Reflector

The problems inherent in refracting telescopes, particularly chromatic aberration and lens sag, can be largely overcome by using mirrors instead of lenses as objectives. A first-surface mirror , with the silvering on the outside so that the light never passes through glass, can be ground so that it brings light to a focus that does not vary with wavelength. Mirrors can be supported from behind, so it is possible to make them larger than lenses without running into the sag problem.

Newtonian Reflector

Isaac Newton designed a reflecting telescope that was free of chromatic aberration. His design is still used in many reflecting telescopes today. The Newtonian reflector employs a concave objective mounted at one end of a long tube. The other end of the tube is open to admit incoming light. A small, flat mirror is mounted at a 45-degree angle near the open end of the tube to reflect the focused light through an opening in the side of the tube containing the eyepiece (Fig. 17-9 A ).

Figure 17-9 A. The Newtonian reflector has an eyepiece set into the side of the tube.

The flat mirror obstructs some of the incoming light, slightly reducing the effective surface area of the objective mirror. As a typical example, suppose that a Newtonian reflector has an objective mirror 20 cm in diameter. The total surface area of this mirror is approximately 314 centimeters squared (cm 2 ). If the eyepiece mirror is 3 cm square, its total area is 9 cm 2 , which is about 3 percent of the total surface area of the objective.

Newtonian reflectors have limitations. Some people find it unnatural to “look sideways” at objects. If the telescope has a long tube, it is necessary to use a ladder to view objects at high elevations. These annoyances can be overcome by using a different way to get the light to the eyepiece.

Cassegrain Reflector

Figure 17-9 B shows the design of the Cassegrain reflector . The eyepiece mirror is closer to the objective than in the Newtonian design. It is not angled, but it is convex. The convexity of this mirror increases the effective focal length of the objective mirror. Light reflects from the convex mirror and passes through a small hole in the center of the objective containing the eyepiece.

Figure 17-9 B. In the Cassegrain reflector, the eyepiece is in the center of the objective mirror.

The Cassegrain reflector can be made with a physically short tube and an objective mirror having a smaller radius of curvature than that of a Newtonian telescope having the same diameter. Thus the Cassegrain telescope is less massive and less bulky. Cassegrain reflectors with heavy-duty mountings are physically stable, and they can be used at low magnification to obtain wide views of the sky.

Practice problems of this concept can be found at: Optics and Telescopes Practice Problems

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