At the eye end the ocular ordinarily consists of two lenses each burnished into a brass screw ring, a tube, flange, cap, and diaphragm arranged as shown in Fig. 29. There are many varieties of ocular as will presently be shown, but this is a typical form. Figure 30 shows a complete modern refractor of four inches aperture on a portable equatorial stand with slow motion in right ascension and diagonal eye piece.
Reflectors, used in this country less than they deserve, are, when properly mounted, likewise possessed of many parts. The smaller ones, such as are likely to come into the reader’s hands, are almost always in the Newtonian form, with a small oblique mirror to bring the image outside the tube.
Fig. 29.—The Eye-Piece and its Fittings.
The Gregorian form has entirely vanished. Its only special merit was its erect image, which gave it high value as a terrestrial telescope before the days of achromatics, but from its construction it was almost impossible to keep the field from being flooded with stray light, and the achromatic soon displaced it. The Cassegranian construction on the other hand, shorter and with aberrations much reduced, has proved important for obtaining long equivalent focus in a short mount, and is almost universally applied to large reflectors, for which a Newtonian mirror is also generally provided.
Figure 31 shows in section a typical reflector of the Newtonian form. Here A is the main tube, fitted near its outer end with a ring B carrying the small elliptical mirror C, which is set at 45° to the axis of the tube. At the bottom of the tube is the parabolic main mirror D, mounted in its cell E. Just opposite the 45° small mirror is a hole in the tube to which is fitted the eye piece mounting F, carrying the eyepiece G, fitted to a spring collar H, screwed into a draw tube I, sliding in its mounting and brought to focus by the rack-and-pinion J.
Fig. 30.—Portable Equatorial Refractor (Brashear).
At K, K, are two rings fixed to the tube and bearing smoothly against the rings L L rigidly fixed to the bar M carried by the polar axis of the mount. The whole tube can therefore be rotated about its axis so as to bring the eye piece into a convenient position for observation. One or more handles, N, are provided for this purpose.