Theory of the Telescope

THEORY OF THE TELESCOPE It is important to realize that magnifying power is not the only quality, perhaps not even the chief quality, desired in a tele scope. We have to pay attention to (I) magnifying power, (2) resolving power, (3) light-grasp, (4) a wide field of good defi nition, and (5) suitability for making accurate measurement. Not all of these can be developed to perfection in the same instrument, and accordingly telescopes of different design are employed for different purposes. For example, the ordinary field-glass is useless for viewing the stage in a theatre; and accordingly an opera-glass is employed which gives a more brilliant though smaller image.

In general, resolving power and magnifying power go together. It is little use making the object appear larger if we do not at the same time sharpen the image so that greater detail may ap pear. We might be content to increase the resolving power (i.e., sharpen the detail in the image) without magnification if that were possible; but optical laws forbid an increase of resolving power without a corresponding increase of magnification. The magnifying power of a telescope can be understood by reference to the theory of geometrical optics alone ; to explain the increase of resolving power, reference must be made to physical optics (the wave-theory of light). Light does not consist of strictly linear rays, but of a wave disturbance which tends to spread and blur any image that is being formed; this tendency to spread is not cured by any perfecting of the figures of the lenses and mir rors, but only by increasing the aperture so that a wider area of wave-front is taken in and concentrated into the eye-pupil. The resolving power of a telescope (free from ordinary optical defects) is simply proportioned to the diameter of aperture of the object-glass or mirror. But in order that the beam filling the object-glass may be narrowed down on emergence from the tele scope so as to enter the eye, the magnifying power must be not less than the ratio of the aperture of the telescope to the aperture of the eye-pupil; if lower power is used an outer ring of the object glass is left unused so that its effective aperture and resolving power are reduced. In looking for faint nebulae we should like to use the large object-glass merely to concentrate more light, and not to dissipate the light again by enlarging the image which it forms ; this, however, is impracticable because we cannot ef fectively use the full aperture of the object-glass without employ ing a corresponding magnification. For this reason comparatively

little progress in our knowledge of nebulae can be made by visual observation, and most of the work is done by photography.

Refracting Telescopes.

In its simplest form the telescope consists of a convex lens (object-glass) which forms an image of a distant object at its focus. This image is then magnified by another small lens (eye-piece) which is used just as an ordinary magnifying glass is used. The object-glass is of long focus F, and the eye-piece of short focus f, and the ratio F/f, is equal to the magnifying power of the telescope. Since eye-pieces are inex pensive it is usual to provide a number of them, so that differ ent magnifying powers can be used; it may be noted that the smallest eye-pieces have the highest power.

The combination just described is Kepler's form of telescope. The course of the rays is shown in fig. 1. In Galileo's form the eye-lens is concave (or negative), and is placed so as to intercept the rays from the object-glass before they reach the focus (fig. 2). This form is common in binocu lars for terrestrial observation, because it gives an erect image, whereas in Kepler's telescope objects are seen upside-down un less a second erecting eye-piece is added. In stellar observation we do not much mind the inversion of the image and Kepler's form is always preferred.

In all modern instruments both the eye-piece and object-glass are compound, consisting of at least two lenses. A single eye-lens would generally give good enough definition at the centre of the field of view, but all the outer parts would be out of focus. Two well-known types of double eye-piece, the Huygens and the Ramsden eye-piece, give a large flat field of view. The construc tion of eye-pieces for the telescope and for the microscope is essentially the same problem. (See MICROSCOPE.) The use of two lenses in the object-glass is necessary in order to correct the defect of colour dispersion already mentioned. In spite of im provements in the manufacture of optical glass, practically the same crown and flint glasses as those used by John Dollond in 1758 are employed in the largest modern telescopes. Owing to what is termed "irrationality of dispersion" no combination of crown and flint lenses will completely get rid of colour dispersion.