Theory of the Telescope

Photography.

When a photograph is to be taken the eye piece is removed and the photographic plate is placed in the focal plane of the object-glass or mirror. It is not a question of adding a camera to a telescope, but of turning the telescope into a camera, the main lens or mirror of the telescope acting as the lens of the camera. In visual observation it is no great drawback if the definition falls off in quality towards the outside of the field of view, because the observer cannot pay minute attention to more than one object at a time; but in astronomical photography it may be a great advantage to have objectives giving good defini tion over a wide field many degrees in diameter. This makes severe demands on the skill of the designer in correcting the various aberrations of the lens system, which usually increase rapidly with the distance from the centre of the field of view. In general, a doublet, consisting of two similar pairs of lenses separated by a wide interval, is employed. The problem of con structing these wide-angle lenses is essentially the same as that of constructing a good camera lens for terrestrial photography (where a wide field is also necessary) ; and indeed a good portrait lens can be usefully employed in celestial photography. The fact that the whole instrument is of insignificant size is irrelevant when speed and not magnification is the main essential. As is well known in terrestrial photography the speed depends on the ratio of the aperture to focal length, and not on the absolute dimen sions of either; and this ratio can be made greater in a doublet than in an ordinary refractor or reflector. For detecting faint diffuse light (e.g., the limits of the tail of a comet, or the ex tended nebulosity in Orion or in the Pleiades) an instrument the size of a hand camera is more effective than a Ioo in. telescope.

Mounting.—If a telescope is kept fixed, the stars in their apparent diurnal rotation pass rapidly across the field of view. Hence for most purposes a telescope should be mounted in such a way that it can automatically "follow" a star. This is contrived by an "equatorial" mounting. Fig. 5 shows the so-called English

form. AA is the "polar axis," which is parallel to the axis of the earth's rotation, and therefore elevated at an angle equal to the latitude of the observatory. By rotating the whole instrument about this axis the effect of the earth's rotation is precisely counteracted, and the telescope remains pointing in the same absolute direction in space, i.e., to the same star. The necessary rotation about AA is given by a driving-clock. The telescope tube is also free to turn about a "declination axis" at right-angles to AA, so that it can be pointed to objects of higher or lower declination; the declination is read off on the graduated circle BB. DD is a similar circle for reading the right ascension or hour-angle.

The English mounting had generally been considered rather antiquated, and all modem instruments were made on a plan which (although the same in principle) avoided the necessity of supporting the polar axis at its upper end ; but its reputation is perhaps rehabilitated by the fact that it was found necessary to revert to it for the i oo in. reflector at Mt. Wilson.

An important adjunct to an equatorial mounting is the driving clock. For visual work no great perfection is required, but for photography it is essential that the image should remain fixed at one point on the plate throughout the exposure. For this purpose the rate of the driving-clock is first controlled by a mechanical governor, which keeps it approximately steady. Then there is an electrical control, which puts in an accelerating or retarding mechanism, according as the clock is ahead of or behind current signals coming once a second from a freely swinging pendulum. Finally, since no automatic control can compensate for the chang ing displacements by refraction as the object rises or descends in the sky, the ultimate correction is made by hand. The observer watches a star seen in a parallel guiding telescope (or by some equivalent device) ; and whenever he detects a tendency to drift away from the original position marked by cross-wires he brings it back by a hand-control.