Telescope

Collier's New Encyclopedia (1921)

an optical instrument to assist the naked eye in examining distant objects. It does this in two ways-by magnifying the apparent angular dimensions of the object, and by collecting more light than the pupil of the eye could alone do to form the image on the retina. One point should be noted that is often misunderstood. If any object is large enough gh to be seen distinctly as a surface by the naked eye, it cannot be made to appear any brighter in a telescope; in fact, it will always be fainter on account of the loss of light by reflection, and absorption in the object glass and eye piece. If, however, the object is faint and rather small to the naked eye, like a distant clock face in the twilight, it can be read more easily in a telescope. If the person should approach the clock face till it subtended the same angular dimensions as its image in the telescope, he could read it more distinctly, and it would appear more brightly illuminated than in the telescope at the first distance, provided he shielded his eyes from surrounding stray light, which is another way in which the telescope sometimes helps in seeing faint objects. If, however, the sources of light examined are from points, like the apparent images of a star, whic which remain practically points under all magnifying powers, then these images will appear more and more bright the larger the aperture of the telescope, provided the whole of the emergent pencil of rays from the object glass enters the pupil of the eye. With large telescopes and low magnifying powers the emergent pencils are generally too large for this. Essential Parts. - The essential parts of a telescope are: (1) Either a concave mirror or a system of lenses, called the objective, for bringing to a point in the focal plane the cone of rays which proceeds from each point of the luminous object and falls upon the objective (for objects at a very great-practically infinite distance these cones of rays that fall upon the objective may be regarded as cylinders). (2) An eye piece consisting of one or more lenses for examining and magnifying the image formed in the focal plane of the objective. (3) A tube or framework of some kind to hold the objective and the eye piece in their proper relative positions. These are all the essential parts of the simple telescope itself, regarded as an optical instrument only. But for the purposes of convenient use many other accessories of mounting and conveniences of handling are required. Moreover, a telescope fitted only as an optical instrument, for looking at objects simply, is of very little scientific value in these modern days of exact measurement and quantitative determination of everything examined, so that in many forms of astronomical, geodetic, and laboratory instruments the optical telescope or microscope is only one of the important parts of the whole, the accessory apparatus ratus attached to it, or to which it is attached, constituting in some instruments the more important feature of the whole. Moreover, for purposes of greater convenience in handling and pointing or of comfort to the observer, many considerable modifications of the optical part itself are introduced, as in the "broken back" transit, where the cone of rays from the objective of the eye piece is reflected at right angles in its course and brought out at one end of the axis, where the observer works in one position without having to follow the revolutions of the eye piece, as in the ordinary form. Also in the equatorial-coudé, or elbowequatorial, first equatorial telescope introduced at the Paris Observatory by Loewy, the observer sits in one position at the upper end of the equatorial axis, which is itself a part of the telescope tube, the rest of it branching off at right angles from the lower end with the objective at the outer end of this branch, with a large mirror in front of this objective turning on an axis which enables the instrument to be pointed to any polar distance, and the motion of the whole round the polar axis tube reaching any hour angle from the meridian. Another mirror at the lower end of this axis reflects the rays up to the focal plane at the upper end, where the observer at the eye piece has the whole instrument under his control, and works as he would at a microscope, having, moreover , this end inclosed in a warmed room built round the eye piece end if he so desires . Reflectors and Refractors. -Two principal forms of astronomical telescopes are in use today, reflectors and refractors. In the first the rays are brought to a focus by falling on a concave mirror; in the second by passing through a system of lenses. In this connection it is worth noting that in French the word telescope generally refers to reflectors, lunette denoting refractors. Though refractors were first invented, no very important astronomical discoveries were made, as we look at telescopic discoveries today, till the invention and use of reflectors by Herschel. There are several forms of reflectors, the Herschelian, in which the mirror is tipped a little in the tube, so that the rays are brought to a focus at the upper end near one side, the observer looking down the tube; the Newtonian, in which a small plane mirror is introduced at an angle of 45°, and reflects the beam out at the side of this tube; and the Gregorian, or Cassegranian , in which the large mirror is pierced in the center and the rays are reflected back through it from a second flat, concave, or convex mirror part way up the tube. In the first two forms the observer must work in an awkward position at the top of the telescope; in in the latter he works at the bottom, as in a refractor, and the telescope lescope is much shorter. Reflectors are cheaper, easier to construct, and can be rendered perfectly achromatic, romatic so that they are equally good for optical, photographic, and spectroscopic work. Here, however, their superiority ends. They are awkward to handle and use, their reflecting power constantly diminishes by tarnish so that they must be frequently ished or resilvered, they require more repolaccurate figuring at first, they do not give so much light for the same aperture, even in their best reflecting condi and , , most important of all, any distortion condition through flexure-they are very heavyor through local effects of temperature, causes almost in the images than the same degree of infinitely more distortion flexure or distortion of a lens; for when lens a bends, one side becomes more convex and the other more concave these nearly counterbalance each other, , and while the effect of bending or distorting a mirror is to deflect the rays just the whole amount. This is why a retwice flector has to be humored and handled in the most careful manner, and special precautions taken in the mounting and support of the mirror; while a refractor, once finished, can be roughly used, and still it will give better definition than the average reflector. A refractor corrected for optical work cannot be used with good results for photography unless a lens is attached in front of the objective, third or one of the lenses of the objective turned over and their distance changed; and, with any construction whatever in a refractor , the spectroscope must be adjusted to a different focal length for different wave lengths of the spectrum. With respect to the distortions produced by flexure, etc., in reflectors, it should be noted that any form of refractor which uses a mirror as a part of its construction is open to the same objection. It is for this reason that telescopes of the form of the equatorial-coudé, in which there are two reflections from mirrors, are not likely to be in demand best definition is a point to be sought where the after. This form of telescope makes an excellent form for a rapid and convenient cometseeker. With the growth in size of their use and management becomes more telescopes difficult, and the mounting has become as important a part as any in construction of large modern telescopes. the In 1918 the second largest telescope (72 inch) ch) was mounted on Little Saanich Mountain, Vancouver Island, B. C. There are three 60-inch telescopes in the Western Hemisphere; one is at the National Observatory, Argentina, another was constructed for Harvard University, at Ealing , England, and another was erected at Mt. Wilson, U. S. A., in 1908. See LENS: OBSERVATORY: LICK OBSERV- ATORY: YERKES OBSERVATORY.