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HANDBOOK OF PHOTOGRAPHY 



resulting has added materially to the service which reflectors of large aperture may 

 render. 



Schmidt Camera. — A very novel type of astronomical camera was announced in 

 1932 by Bernhard Schmidt of the Hamburg-Bergedorf Observatory. The Schmidt 

 camera is an ingenious combination of a spherical mirror with a thin lens of peculiar 

 curvature which is placed at the center of curvature of the mirror. The lens elimi- 

 nates the spherical aberration introduced by the mirror and makes possible a con- 

 struction of a camera of extremely short focal length and wide aperture ratio. 



Perhaps the most notable of Schmidt cameras was that recently installed at Mount 

 Palomar in California which is the site for the projected 200-in. reflector. In this 

 particular telescope the spherical mirror is of 24-in. aperture and the correcting 

 lens 18 in. in diameter. Since the lens is at the center of curvature of the mirror, it 



Fig. 3. — Eighteen-in. Schmidt telescope on Palomar Mountain, Calif. 



occupies a place at the upper end of the tube of the camera. The plate is placed at 

 the focus of the mirror, halfway between the surface of the mirror and the thin lens. 

 The mirror is necessarily made larger than the lens so that the 18-in. beam of light 

 from the stars off the axis may be fully reflected from the surface of the mirror. The 

 Schmidt arrangement, therefore, makes possible a much wider effective field than 

 can be obtained from either a mirror or a lens alone and has the advantage of permit- 

 ting a focal ratio in this instance of //2. It was with this instrument that the two 

 notable supernovae of 1937 were discovered by Zwicky. 



The ingenuity of design of the Schmidt arrangement makes possible a focal ratio 

 of even //I. Such a camera of only 4-in. focal length has been constructed for the 

 Ladd Observatory of Brown University under the direction of C. H. Smiley. Mention 

 should here be made of a 48-in. Schmidt camera which is now being planned for the 

 Mount Palomar Observatory. 



While the Schmidt arrangement has the great advantage of large aperture and 

 exceptional speed, it is not without some disadvantages. The extremely short focal 



