The Principal Focus
As already indicated, the principal mirror when used alone forms an image of the star 30 feet above, at the centre of the upper end of the tube, and an eyepiece could be placed there for visual work or a photographic plate for direct photographs of nebulae, etc. However, it is generally more convenient to use a flat mirror at 45° forming the image at the side of the tube for photographs and so the telescope is only used in this form with a small spectrograph for the ultra-violet region of star spectra. The course of the parallel beam of light from the star to its image on the slit of the spectrograph is graphically shown in Fig. 5 A and also its passage through the slit prisms and lenses of the spectrograph. The position of the star image on the slit of the spectrograph can be observed by a guiding telescope extending to the edge of the tube and can be kept central by the portable aluminium switchboard already described. The elevating platform is of course used in work in this position.