Carol Worth, Age 11., of Kentfield, Calif., for her question:
How do refracting and reflecting telescopes differ?
Refracting telescopes were invented first, and the biggest of them is in the Yerkes Observatory in Wisconsin. It scans the skies with a glass lens 40 inches wide. Reflecting telescopes are far more powerful, and the granddaddy of them all is housed high on Mt. Palomar in California. Its mirror bright, round. Eye is 200 inches wide.
A telescope is a clever gadget for making faraway things look near. It does the trick with curved glass lenses or mirrors or with both. A simple refracting telescope refracts or bends light rays through glass lenses. One end of the tube is pointed at a distant object and the Viewer looks through an eye piece at the opposite end. A reflecting telescope has mirrors for reflecting the light, and. In most of these instruments the eye piece is on the side of the tube.
A refracting telescope may be simply two glass lenses fitted in a metal tube, one at each end. The tube is in two sections, one inside the other. It can be adjusted to move the lenses closer or farther apart for focusing on a distant object. The larger lens, at the wider end of the tube, gathers light frown, say, a star and bends the beams together in a small, vivid image. The eye piece magnifies the small, concentrated image.
A reflecting telescope can be much bigger. Light frown afar is gathered by a concave mirror which is set in the bottom of a wide tube. A small, flat mirror is set inside the tube near the top and. Tilted at an angle of 45 degrees. In this telescope, the eye piece is set in the side of the big tube, near the bottom.
The image gathered by the big, bowl shaped mirror is reflected up to the little flat mirror inside the tube. From there it is reflected o the eye piece, where it is magnified. Some reflecting telescopes have a round hole in the center of the big mirror.
The eye piece is behind the hole at the bottom of the tube. Halfway up the tube is a small, curved mirror. Light from a star is gathered by the big mirror, sent up the tube to the small mirror and reflected down again into the eye piece.
As the earth turns, the heavenly bodies seem o move over the sky. A large telescope of any type is controlled by clockwork machinery which keeps it trained on a moving object. Cameras can be fitted to the eye piece, and many of the photographic plates need long exposures. The work of the big reflecting telescope is all done by photography. Long exposures of the far distant skies are made on plates and later studied by the astronomers.