Telescope

Telescope, (from two Greek words meaning to view after) is employed as a means of enabling us to see magnified objects. Telescopes may be divided into two classes: (1) those in which the first image is produced by a lens - these are known as refracting telescopes; (2) those in which this image is produced by a concave mirror - these are known as reflecting telescopes.

The simplest of the refracting instruments is the astronomical telescope. This consists of a convex lens or object-glass O, and a convex eye-piece E. If a very distant object A C B be viewed through this telescope, each point in it will send parallel rays to the lens o. The parallel rays from A will converge to the point a, and those from B will converge to b. A real inverted image a c b will thus be formed of the object A B, and this image will be at the principal focus of the object-glass. The image a c b can now be considered as the object viewed by the lens E, and if E be adjusted so that its distance from a b is rather less than its own focal length, a virtual image a'c'b' will be formed larger than a c b, and farther away from the lens. This enlarged image will be viewed by the eye placed behind E. In the diagram (Fig. 1), to avoid confusion, only those rays are drawn which are parallel to the axis and are sent out by the point c in the object. The dotted lines indicate that the rays do not actually pass through e', but only appear to come from that direction. The distance between the two lenses is almost exactly the sum of their focal lengths, and the magnifying power is their ratio. In practice the lenses are mounted in tubes, and may be brought nearer together or farther apart to suit the convenience of the observer, and to be adjustable for viewing terrestrial objects, which are not, of course, always equally far off. The size of the object-glass determines the amount of light which shall enter the eye, and this must be so great in order to get a sufficient illumination of a very large image that it practically fixes a limit to the amount of magnification obtainable, or at least introduces enormous difficulties in the way of obtaining extremely high magnification.

In astronomical observations the inversion of the image is not of any importance; but when terrestrial objects are viewed it is not, as a rule, desirable to see them upside down. To avoid this, two equal convex lenses are often placed between a b and E; this reinverts the image which is seen through E, so that the final virtual result is erect. Other arrangements may be used instead of the convex lenses to produce the same effect.

In the Galilean telescope inversion of the image is avoided by the substitution of a concave for the convex eye-piece.

Rays from the object would, after passing through the object-glass O, naturally form the real inverted image a b; the rays are, however, confronted on their way by the lens E placed at a distance rather more than its focal length from a b, which makes them more divergent. the outer ray a b is bent upwards into the eye, to which it appears to have come from a point b', while the ray O E is turned so that it seems to have come from a'. The virtual image a' b' is therefore erect. The magnification, as before, is the ratio of the focal length of the object-glass to that of the eye-piece, while the distance between the two lenses is the difference of their focal lengths. This makes it a much more convenient size for ordinary use, and hence opera and field-glasses are constructed on this principle, they being, in fact, merely a pair of Galilean telescopes. It is usual to make both object and the component parts being of different kinds of glass. [LENS.]

The Galilean telescope could not be used for measuring the exact position of an object for cross lines would be ineffective, since there is no real image formed with which they could coincide.

In reflecting telescopes, the convex object-glass is replaced by a large concave mirror S, and would form a real inverted image at a b; these rays are, however, reflected by a plane mirror M placed across their path, so that the image is formed at a' b' instead. Viewed by an eye-piece E, this last image appears magnified at a" b". The magnifying power is expressed by the ratio between the focal length of speculum and eye-piece.

In the Gregorian - the first reflecting telescope to be invented - the speculum contains a circular hole in its centre; it forms a real image in the same position as a b in the Newtonian telescope. A second but small concave mirror, suspended in the centre of the tube, with its back to the light, forms a second real image in front of itself. This image, being the inversion of the first, is erect with regard to the object. The second mirror is so placed that the second image falls near the aperture in the centre of the speculum; an eye-piece placed behind the hole is then used as a means of magnifying it again. The magnifying power is approximately equal to the ratio between the square of the focal length of the speculum to the product of focal lengths of small mirror and eye-piece.

Cassegrain's differs from Gregory's telescope in that the first image a b is not actually formed; the rays are intercepted by a convex mirror, which forms an image, as before, near the aperture in the speculum, and this is viewed by a similar eyepiece. The distance of this convex mirror from the position which a b would occupy is rather less than its focal length. The approximate magnifying power is expressed by the same ratio as that which held for Gregory's. This telescope has the advantage of being shorter than Gregory's and of partially correcting spherical aberration [ABERRATION]; but for terrestrial observation it is not so convenient, since it gives an inverted image of the object.

Most telescopes are provided with a compound eye-piece, the usual form being that invented by Huyghens. This consists of two convex lenses, the focal length of the one farthest from the eye being three times that of the other, while the distance between the two is the difference of their focal lengths.

Achromatic refracting telescopes give much brighter and more distinct images than are obtained from similar reflectors, but the difficulties in the way of making large object-glasses are enormous. The necessary thickness of the lens makes it very liable to have stresses and strains set up inside it, so that the path of some of the rays is distorted and the image is blurred. Specula, on the other hand, can be made enormously big; Lord Herschell's, in fact, was so large that he could directly view with an eye-piece the real image formed by it. His face was thus turned directly towards the speculum, the fact that his head was in the path of the initial rays not causing any serious interference.

The famous reflecting telescope of Lord Rosse, erected in 1844, has an aperture of 72 inches, while that of the largest refractor - that in the Yerkes Observatory at Chicago - measures 40 inches. The refracting telescope at Greenwich has an aperture of 28 inches.