[CHAPTER VIII]
ACCESSORIES
Aside from the ordinary equipment of oculars various accessories form an important part of the observer’s equipment, their number and character depending on the instrument in use and the purposes to which it is devoted.
Fig. 126.—Star Diagonal.
First in general usefulness are several special forms of eyepiece equipment supplementary to the usual oculars. At the head of the list is the ordinary star diagonal for the easier viewing of objects near the zenith here shown in Fig. 126. It is merely a tube, A, fitting the draw tube of the telescope, with a slotted side tube B, at a right angle, into which the ordinary ocular fits, and a right angled prism C with its two faces perpendicular respectively to the axes of the main and side tubes, and the hypothenuse face at 45° to each. The beam coming down the tube is totally reflected at this face and brought to focus at the ocular. The lower end of the tube is closed by a cap to exclude dust.
One looks, by help of this, horizontally at zenith stars, or, if observing objects at rather high altitude, views them at a comfortable angle downward. The prism must be very accurately made to avoid injury to the definition, but loses only about 10% of the light, and adds greatly to the comfort of observing.
Of almost equal importance is the solar diagonal devised by Sir John Herschel, Fig. 127. Here the tube structure A, B, is quite the same as in Fig. 126 but the right angled prism is replaced by a simple elliptical prism C of small angle, 10° or less, with its upper face accurately plane and at 45° to the axes of the tubes, resting on a lining tube D cut off as shown. In viewing the sun only about 5% of the light (and heat) is reflected at this upper surface to form the image at the eye piece.
