The Adjustment Testing Of Telescopic Objectives PDF Download

Excerpt from On the Adjustment and Testing of Telescopic Objectives In the first place it may be pointed out that portable and pocket telescopes of under three inches aperture and used for terrestrialpurposes, are adjusted by their makers before being sent out, and moreover are not usually supplied with apparatus for adjustment, as the comparatively low magnifying powers used on such telescopes renders a perfect adjustment of the objective unnecessary. Of course, good objectives for astronomical purposes, if supplied with tube and mounting, are also sent out in good adjustment, as far as possible; nevertheless, after they have been in use for some time, or the objective has been taken out for cleaning, &c., or still more, if they have changed hands and undergone vicissitudes of some sort or another, there will arise need for more or less readjustment of the objective with respect to its tube, or squaring on as the operation is called. Sometimes the maker has to supply a large telescope to be put up at some distant place (in Australia, for instance), in which case it may be impracticable to send some well qualified person to see to the final adjustment of the objective in its tube; or again, an objective may be supplied by itself, in which case its adjustment by the maker is not demanded, but is undertaken by its owner or a friend. It is therefore very important that anyone who uses an astronomical telescope of any considerable aperture should be able to perform the operation of squaring on the objective to a nicety, for unless this is done the telescope will not perform its best with the highest magnifying powers, especially in the'way of separating difficult double stars. Technically, the operation called squaring on means the adjustment of the optical axis of the objective until it passes accurately through the centre of the eye-piece. If the objective is not squared on, its Optical axis falls more or less to one side of the centre of the eye-piece, and we have the condition of things shown in Fig. I, where o-a is the optical axis of the objective, falling to one side of the eye - piece, so that the latter is looking obliquely at the objective, as it were. It is impossible for all the rays passing through the objective to focus accurately to a point on the oblique axis o-a, whatever the type of objective may be, although, as will be pointed out further on, there is one type which renders possible a nearer approach to good definition on an oblique axis. If the objective is squared on the Optical axis ma will now be transferred to the position passing centrally through the eye - piece. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.