Page:Popular Science Monthly Volume 89.djvu/922

 Studying the Stars with Mirrors

��The biggest reflecting telescope in the world belongs to Canada

By Dr. C Furness Professor of Astronomy in Vassar College

��IT IS with the reflecting tele- scope that many of the most brilliant discoveries about stars are made. Us construction, how- ever, is not so gen- erally understood as that of the refract- ing telescope, the form of instrument which is so often seen in the parks or on the streets of our cities and through which the passerby can get a peep at the Moon for the trilling sum of five or ten cents. By calling attention first to certain facts regarding this more familiar type of telescope, it will be

easier to make clear the construction of the reflecting telescope.

The lens at the upper end of a refracting telescope is called the object glass. It collects the rays of light and brings them together at a focus to form an image, which is viewed with a magnifNing eye- piece. The largest refracting telescope is the well-known Yerkes instrument. It has an object gla.ss forty inches in diameter.

In a reflecting telescope, the light is col- lected by reflection from the surface of a concave mirror. If this surface is ground to a parabolic shape, the ra\s will all come together at a single point to form an image, just as with the refracting telescope; but this point will be situated on the same side of the mirror as the object, and hence the observer who tries to look at a star will find his head in his own line of vision. In order to overcoi7ie this dilficully, a second reflection is made to lake place, so as to

����Above : Spiral nebula Messier 101, Ursae Majoris, photographed with the two - foot re - flector of the Yerkes Observatory. Time of exposure, three hours

Another photo- graph of the same nebula, taken with the sixty-inch re- flector of the Mt. Wilson Solar Ob- servatory. Time of exposure, seven hours, thirty min- utes. Comparison shows the greater detail in the lower photograph

��deflect the beam of light and form the image at one side of the tube, where it may easily be examined with an eyepiece. This second reflection is accomplished b\- means of a ])Iane mirror or "flat" inserted in the upper end of the tube and set at an angle of 45°. This flat will neccssarih- cut oft' some of the light falling u|Xin the (principal mirror, but since it is not large and since its sujjports are made as slender as possible, there is no serious loss.

A Mirror Six Feet in Dia meter

At first mirrors were made of speculum metal, an allo\- of copper and tin, which can i)e very highly ])olished. As early as 1842, the famous reflector of Lord Rosse was constructed. It had a mirror six feet in diameter. With this instrument many dr.iwings of nebulae and planets were made. Howexer, it never attained the usi-fiilness wliirli might have been expected, chielh' on

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