Telescopes have been very important in the history of Astronomy. Since 1609 when Galileo and Thomas Harriot first started using telescopes in astronomy, these contraptions have blossomed our knowledge of the unknown, the universe.

Lenses

A telescope uses two or more lenses. Lenses bend the light from a given source in such a way to enhance the image of the object. A lens is either convex or concave. A convex lens curves out, while a concave "caves" in.

		Concave Lens			Convex Lens

The aperture of a lens is its diameter. The focal length is equal to the distance of the lens from the focal point, where the lens brings the light rays to a focus.

Telescopes sometimes have two or more lenses. The lens(es) near the eye of the observer is(are) part of the eyepiece of the telescope, while the lens at the other end of the telescope is called the objective lens.

Lenses sometimes produce distortions in the image of the object viewed. Two distortions are known as chromatic aberration and spherical aberration.

Chromatic aberration results from the fact that a curved transparent lens bends differently colored light by differing amounts. Any given glass lens will possess different indices of refraction for rays of different colors, producing the result that rays of different color, even when coming from the same point of the object in view, will be bent differently. Thus a focus at one point, blue rays at another, will blur the image of the object.

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Spherical aberration results from the fact that most lenses have surfaces that are spherical in shape; this is because it is easier to shape lenses into spherical form than hyperbolic or parabolic form. Because of spherical aberration, parallel rays striking the lens at different distances from its center are brought to a focus at different points. This results in a blurring of the image seen. This aberration can be reduced by grinding the surface in a parabolic form and also by making telescopes of very long focal length. Long focal length also helps reduce chromatic aberration, but to a lesser extent.

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Three Main Types of Telescopes

Although other types of telescopes eventually evolved, nearly all pre-20th century telescopes can be classified in terms of three design types: the Galilean telescope, the Keplerian telescope, and the reflecting telescope. The Galilean telescope, sometimes called an opera glass, consists of a concave eyepiece and a convex objective lens. In 1608, Hans Lipperhey, a Dutch lens maker, applied for a patent for such a telescope. In 1609, Galileo, learning of the Dutch instrument, constructed his own telescope and became the first to use it extensively for astronomical observation. Galilean telescopes give an erect image and are usually shorter than keplerian refracting telescopes, mainly because the eyepiece lies within the focal length of the main lens, whereas the eyepiece lies beyond the focal point in the Keplerian. Galilean telescopes are very limited in size and usefulness, but provide partial correction for both spherical and chromatic aberration. By the end of the 17th century, the best telescopes no longer employed the Galilean optical system.

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The Kaplerian or astronomical telescope consists of a convex eyepiece and a convex objective lens. Johannes Kepler in 1611 first designed this type of telescope, which was first constructed by Christopher Scheiner in the 1613-17 period. It produces an inverted image, which is not a problem for astronomers, but it is for anyone watching a football game or an opera. It can be made in large size and is the design found in essentially all major post-17th century refractors. Such telescopes suffer from spherical and chromatic aberrations, but astronomers gradually learned to correct for these problems.

The primacy accorded refracting telescopes during the 17th century began to be challenged late in the 18th century by the reflecting telescope, the first of which was constructed by Isaac Newton in 1668, although the idea of making a telescope with an objective lens consisting of a mirror had been proposed earlier. Mirrors, like traditional lenses, bring rays to a focus. In the Newtonian focus reflecting telescope, the rays enter the remote end of the tube, proceed down the tube until they strike spherically or parabolically curved concave mirror, which reflects the rays back to a second, planar mirror, from which they are reflected to the eyepiece positioned on the side of the tube.

Three Types of Reflecting Telescopes

One reason why Newton developed the reflector is that no chromatic aberration occurs in reflection; thus reflecting telescopes solve one of the most severe problems of telescope design. Besides, for a mirror, only one surface must be ground, whereas in a refracting telescope, both sides of the objective lens are usually figured. Also reflecting telescopes can be made very large; the largest mirror presently used is nearly eight times larger in diameter than the largest transparent lens. Until after about 1860, the mirrors of nearly all reflecting telescopes were metal, making them both very heavy and more subject to distortions resulting from thermal contraction and expansion than glass lenses. Reflectors come in a number of designs in addition to the form designe by Newton; among these are the Cassegrainian and Herschelian focus types.

In the Cassegrainian design, the second mirror is curved and the eyepiece is placed behind the telescope mirror, in which a small hole is cut so as to allow the rays to pass through to the eyepiece. Cassegrainian telescopes possess the advantage that astronomers observe from the telescope's lower end, rather than its remote end. Reflectors of Herschelian design have the advantage that a second mirror is not needed, thus conserving the amount of light lost in any reflection. This design was created by Sir William Herschel in the 1780s, but is rarely used today.

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Some Factors that influence the quality of a Telescope

1. Light-Gathering Power and Aperture

The most important criterion of the quality of a telescope is its power to gather light. This depends on its aperture - the diameter of its main lens or mirror. The difficulties we experience in trying to see a dim object at night are due to the fact that it supplies insufficient light to register on our retina. We compensate for this by expanding the apertures of our eyes. Similarly, astronomers seek to improve their capability for observing distant objects by building telescopes of large aperture. The mathematics of this is that light-gathering power increases in proportion to the aperture squared. Let us set the light-gathering power of the eye as 1 and assume that the eye has an aperture of 0.2 inches. Using this data, calculate the light-gathering power for each of the five historic telescopes listed below.

• 1609 Galileo's refracting telescope (a = 3/2 inches) ___56___
• 1789 Herschel's 48-inch aperture reflector ________
• 1907 Mount Wilson Observatory 60-inch reflector ________
• 1948 Palomar Observatory 200-inch reflector ________
• 1976 Crimean Observatory 236-inch reflector ________

2. Focal Length

Another important criterion of the quality of a telescope is focal length, which is the distance between its objective lens and the image formed by that lens. The focal length of the objective lens is the most important factor influencing magnification in the telescopes. The formula for the magnification of a telescope is the focal length of the objective lens divided by the focal length of the eyepiece. Thus if one telescope has twice the focal length of another, it should magnify twice as much, provided that the same eyepiece is used with both telescopes. These facts should make flear that magnification by itself is not a good criterion of the quality of a telescope. The reason is that essentially any magnification can be attained with any particular telescope, simply by employing an eyepiece of sufficiently short focal length. The problem is that telescopes with short focal length objective lenses give far less useful magnification that those of longer focal length. Assuming that an eyepiece of focal length 4 inches is applied to each of the telescopes listed below, calculate the magnification that would result.

• Galileo's telescope (focal length = 3 ft.) ______
• Herschel's largest telescope (f = 40 ft.) ______
• Mount Wilson Observatory telescope (f = 25 ft.) ______
• Palomar Observatory telescope (f = 55 ft.) ______
• Crimean Observatory telescope (f = 100 ft.) ______

3. Resolving Power

Because of diffraction and other factors, the image of a star or other small object is spread so as to form a small circle. Suppose we have two small star images, or two images from a surface feature on a moon or planet, and suppose that these images are very near each other. If the circles associated with them are too large, the circles will merge so that we see one circle or near circle rather than two separate images. Such distortion is obviously undesirable. The measure of a telescope's ability to resolve fine detail is called its resolution or resolving power and is mesured by the limiting arc distance between two objects at which they can still be seen as separate. For example, astronomers test telescopes by seeing how small the angle of separation between two stars can be without preventing the separate stars from being seen as separate. The higher the quality of the telescope, the smaller is its resolving power. For visual telescopes, resolving power is equal to 4.56" divided by the aperture measured in inches. Compare the resolving power of the five telescopes listed previously.

• Galileo's refracting telescope (a = 3/2 inches) ______
• Herschel's 48-inch aperture reflector ______
• Mount Wilson Observatory 60-inch reflector ______
• Palomar Observatory 200-inch reflector ______
• Crimean Observatory 236-inch reflector ______

Note: Other factors also influence telescope quali