Theory of the Telescope

In some of the newer glasses the irrationality of dispersion has been reduced to a minimum, but they have nothing approaching the same difference of dispersive power as the ordinary crown and flint glass (a difference of i to ii) ; deeper curves and thicker lenses would be required, which would neutralize all the advan tages of using them. Accordingly in practice the achromatism is made as perfect as possible for the part of the spectrum that will chiefly be employed ; viz., near the yellow light for visual work, and in the blue and ultra-violet for photographic work. For this reason separate telescopes are used for visual and photo graphic work, respectively.

By using three lenses to form the object-glass photo-visual telescopes can be made, sufficiently achromatic for all parts of the spectrum. The additional surfaces and thickness of glass, however, involve some loss of light ; and the combination has the great drawback that its focal length changes very rapidly when the tempera ture drops at nightfall.

Reflecting Telescopes.

The following are the various forms of reflecting tele scopes: The Gregorian telescope is represented in fig. 3. A A and B B are concave mirrors having a common axis and their concavities facing each other. The focus of A for parallel rays is at F, that of B for parallel rays at f—between B and F. Parallel rays falling on AA converge at F, where an image is formed; the rays are then reflected from B and converge at P, where a second and more enlarged image is formed. Gregory himself showed that, if the large mirror were a segment of a paraboloid of revolution whose focus is F, and the small mirror an ellipsoid of revolution whose foci are F and P respectively, the resulting image will be plane and undistorted. The image formed at P is viewed through the eye-piece at E, which may be of the Huygenian or Ramsden type. The practical difficulty of constructing Gregorian tele scopes of good defining quality is very considerable, because if spherical mirrors are employed their aberrations tend to increase each other, and it is extremely difficult to give a true elliptic figure to the necessarily deep concavity of the small speculum. Sta,rt appears to have systematically conquered this difficulty, and his Gregorian telescopes attained great celebrity. The use of the Gregorian form is, however, practically abandoned.

The Cassegrain telescope differs from the Gregorian only in the substitution of a convex hyperboloidal mirror for a concave ellip soidal mirror as the small speculum. This form has two distinct advantages: (I) if spherical mirrors are employed their aber rations have a tendency to correct each other; (2) the instrument is shorter than the Gregorian, caeteris paribus, by twice the focal length of the small mirror. For spectroscopic purposes the Cas segrain form has peculiar advantages, because in consequence of the less rapid convergence of the rays after reflection from the convex hyperboloidal mirror, the equivalent focus can be made very great in comparison with the length of the tube. This permits the employment of a spectroscope furnished with a col limator of long focus.

The Newtonian telescope is represented in fig. 4. A A is a concave mirror whose axis• is aa. Parallel rays falling on AA converge on the plane mirror BB, and are thence reflected at right angles to the axis, forming an image in the focus of the eye-piece E. The surface of the large mirror should be a para boloid of revolution, that of the small mirror a true optical plane. When used for photography the inclined mirror is re moved ; and the photographic plate is supported in a carrier mounted at the principal focus of the large mirror.

Most modern reflectors are made so that they can be used alternatively in the Cassegrain or Newtonian form by inserting alternative small mirrors. In the earlier reflectors the mirrors were made of metal. The art of making specula was notably developed by Sir William Herschel, who may be said to have created the fame of the reflector. His greatest telescope, made in 1789, was of 4 ft. aperture and 40 'ft. focal length. In Lord Rosse erected the Parsonstown reflector of 6 ft. aperture. These suffered from the great drawback that whenever the mirror became tarnished, the re-polishing was likely to spoil the figure of the mirror. To preserve the quality the figuring must be done all over again, and the telescope became virtually a new instru ment with different optical behaviour. In modern practice the reflecting surface is a film of silver deposited on a glass disc ground to the proper curvature. When the surface loses its bril liance the silver film is washed off (once or twice a year) and a new film deposited.