Astronomical Instruments 2

The Reflecting Telescope

This instrument differs radically from the refracting one already described. It receives the light in a concave mirror, M (Fig. 14), which reflects it to the focus F, producing the same result as the lens of the refracting telescope. Here a mirror may be placed obliquely, reflecting the image at right angles to the eye, outside the tube, in which case it is called the Newtonian telescope; or a mirror at R may be placed

perpendicularly, and send the rays through an opening in the mirror at M. This form is called the Gregorian telescope. Or the mirror M may be slightly inclined to the coming rays, so as to bring the point F entirely outside the tube, in which case it is called the Herschelian telescope. In either case the image may be magnified, as in the refracting telescope.


Fig. 14.—Reflecting Telescope.

Reflecting telescopes are made of all sizes, up to the Cyclopean eye of the Subaru telescope, which is 327 inches, in diameter. The form of instrument to be preferred depends on the use to which it is to be put. The loss of light in passing through glass lenses is about two-tenths. The loss by reflection is often one-half. In view of this peculiarity and many others, it is held that a twenty-six-inch refractor is fully equal to any six-foot reflector.

The mounting of large telescopes demands the highest engineering ability. The whole instrument, with its vast weight, with its accompanying tube and appurtenances, must be pointed as accurately
as a rifle, and held as steadily as the axis of the globe. To give it the required steadiness, the foundation on which it is placed is sunk deep in the earth, far from rail or other roads, and no part of the observatory is allowed to touch this support.

When a star is once found, the earth swiftly rotates the telescope away from it, and it passes out of the field. To avoid this, clockwork is so arranged that the great telescope follow the star by the hour, if required. It will take a star at its eastern rising, and hold it constantly in view while it climbs to the meridian and sinks in the west and the reflector demands still more difficult engineering.

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