Astronomy for Everybody/Part 2/Chapter 2

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Although the refracting telescope is that in most general use, there is another form of instrument of radically different construction. Its main feature is that the functions of the object-glass are performed by a slightly concave mirror. That such a mirror reflects parallel rays falling upon it to a focus, is doubtless well known to our readers. The focus is situated about half-way between the mirror and its centre of curvature.

This form of instrument has an enormous advantage in its freedom from the "secondary aberration" which we have already described as inherent in the refracting telescope. Another advantage which it possesses is that it can be made of larger dimensions than the other. The extreme limit so far reached in the refractor, as we have already stated, is four feet; but the forty-inch aperture of the Yerkes telescope is, up to the present time, the limit in actual use for astronomical research. But, more than half a century ago. Lord Rosse constructed his great reflector of six feet diameter. Judging by its size alone, this instrument ought to give several times more light, and therefore show far minuter stars, than any refracting telescope yet made. But, for some reason, its performance—and, indeed, that of reflectors generally—has not corresponded to the size.

​The practical difficulties in using a reflector are several in number. The first and most obvious one is that the rays are reflected back in the direction from which they came. To see the image the observer must look into the mirror as it were. If he does this directly, his head and shoulders will cut off the light that falls on at least the central regions of the mirror. Some contrivance for reflecting this light away is therefore necessary. Two ways of doing this are in use. In what is known as the Cassegranian reflector, a smaller, slightly convex mirror is interposed between the focus and the principal mirror. An opening is made in the centre of the latter, through which the rays are reflected back by the smaller mirror. The curvature and positions of the two are so adjusted that the image of the distant object shall be formed in this opening. The only telescope of this kind in actual use is the great Melbourne reflector, of four feet diameter, made by Sir Howard Grubb, of Dublin.

The contrivance most in use was designed by Sir Isaac Newton. It consists of a diagonal reflector, which may be a mere glass prism, placed just inside the focus. Its reflecting surface makes an angle of forty-five degrees with the axis of the telescope, and therefore reflects the rays laterally to the side of the tube. Here they are observed with an ordinary eyepiece. This instrument is known as the Newtonian reflector.

It is remarkable that, notwithstanding the immense improvement in the mechanical processes necessary in constructing and mounting a reflecting telescope, no attempt has ever been made to even equal Lord Rosse's ​

​great instrument in dimensions. The largest mirrors so far successfully made and used have been about four feet in diameter. About fifty years ago, Mr. Lassell made one of this size, with which he discovered two new satellites of Uranus. More recently, Mr. A. A. Common, F.R.S., has constructed a mirror of the same size. This has been used in taking photographs of nebulæ and other faint objects, for which this form of telescope seems well designed.

The great difficulty in using a large mirror is that it bends under the influence of its own weight. It would seem that when the diameter exceeds four feet, no way of completely avoiding this difficulty has yet been put into successful use. A mirror of five feet diameter is, however, being made at the Yerkes Observatory by Mr. Ritchie, in which, it is hoped, all the difficulties will be surmounted.

In the instruments of Lord Rosse and Mr. Lassell, the mirror was made of an alloy, known as speculum metal. Recently, however, the use of speculum metal has been superseded by another arrangement. The concave mirror is made of a large disk of glass, which is ground and polished into nearly spherical form, or to speak more accurately, a parabolic form, because the latter is necessary to bring all the rays to one focus. A thin coating of silver is then deposited on the surface of the glass, which is susceptible of a high polish, and reflects more light than polished metal.