294 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



display as is evidenced by the accompanying photographs. The 

 fastest photographic emulsions, however, utilize only about 1 percent 

 of the incident light. The best photoelectric devices available to the 

 astronomer today can utilize approximately 30 percent of the light. 



TELESCOPES 



The principal auxiliary instruments currently used on a telescope 

 are the photoelectric photometer and spectrophotometer, the photo- 

 graphic spectrograph, the photographic camera, and recently a new 

 device, the image intensification tube. A spectrograph is a most power- 

 ful device in the hands of the astronomer. It can reveal many 

 interesting properties of a star, such as temperature, mass, chemical 

 composition, the extent of its atmosphere, whether the star is single, 

 a binary system, etc., its motion toward or away from the observer and 

 indirectly, but effectively, the absolute brightness, distance and even 

 the age of the star. I cannot go into the many interesting details of 

 how each of these things is learned, but I hope that the recitation of 

 this list will give the reader a glimpse into the fascinating world open 

 to the astronomer. 



The astronomer has pushed the telescope close to its maximum useful 

 size in the current century. At this time we have four telescopes 

 that have been built with mirror diameters of 100 inches or more. 

 In order of size these telescopes are the 200-inch Hale telescope on 

 Mount Palomar, completed in 1947 ; the 120-inch on Mount Hamilton, 

 in 1959 ; the 104-inch now being placed in operation in the U.S.S.R. : 

 and the famed 100-mch on Mount Wilson, completed in 1919 and 

 which did much to revolutionize observational astronomy in the hands 

 of Hubble and Baade. A giant telescope of 240-inch aperture and 

 of revolutionary design is reported under design by the Institute of 

 Optics in Leningrad. A new telescope of 150-inch aperture has also 

 reached the design stage at the Kitt Peak National Obser\^atory 

 in the U.S.A. 



Since new and larger telescopes cost much in return for a relatively 

 small gain in distance reached and in new knowledge, astronomers are 

 now concentrating upon the use of new methods to make better use 

 of the starlight that is collected by our terrestrial telescopes. They 

 are also looking forward to the utilization of space telescopes, but 

 before I speak of these new telescopes it is necessary to appreciate the 

 handicaps with which the astronomer is faced with his telescopes 

 located upon the surface of the earth. 



The atmosphere limits the usefulness of a large telescope even on 

 the clearest night atop a mountain. The small turbulences present 

 in the atmosphere that are accompanied by thermal differences make 

 it impossible to sharply focus a telescope. The air itself will not 

 permit the far ultraviolet light or the infrared light to reach the 



