ASTRONOMICAL TECHNOLOGY — ^MEINEL 295 



highest mountain. In addition, the night sl^ background is not 

 entirely dark. Far fi'om the lights of the city and on a moonless 

 night one can see with the dark-adapted eye well enough to read the 

 large headline type of a newspaper. The stars are therefore seen by 

 a telescope as upon this faintly luminous background. This diffuse 

 "light of the night sky" and the lack of sharpness of focus from at- 

 mospheric turbulence combine to set the limit in faintness to which a 

 telescope can reach. 



The total briglitness of the night sky backgi'ound with a large tele- 

 scope is approximately equal to that of one 20th magnitude star per 

 square second of arc. It is obvious, therefore, that Ave cannot tolerate 

 many square seconds of arc in a detector when we wish to observe a 

 23d magnitude star. Since the light of the night sky is a diffuse 

 source, its brightness does not depend upon the mirror diameter of 

 the telescope but only upon the ratio of the focal length divided by 

 the aperture, called the f-ratio or f-number of tlie telescope. 



INCREASING THE EFFICIENCY 



Faced with the above limitations the astronomer has four means 

 at his disposal where gains can be made as follows : 



1. Increase the size of the telescope mirror, 



2. Increase the efficiency of the detector, 



3. Decrease the aperture, and thereby the night sky noise at the 

 detector, 



4. Place the telescope above the atmosphere, either on a high balloon 

 or a satellite. 



The first alternative, building a larger telescope, has been con- 

 sidered. Wliile it is within the scope of present teclmology to build 

 a 400-inch telescope, its cost would be in the vicinity of $40 million. 

 Its ultimate benefits would be doubtful in terms of the great expense 

 because of the limitations imposed by atmospheric seeing unless a 

 site with unprecedentedly fine seeing could be found. 



The term "seeing" is used by the astronomer to refer to two dis- 

 turbances caused by the atmosphere. They are (a) time fluctuations 

 in the intensity of the wavefront arriving at various points at the 

 telescope aperture, and (b) time fluctuations in the direction of arrival 

 of the wavefront. The first is called "scintillation" and is readily 

 seen with the unaided eye as twinkling. The second effect is usually 

 referred to as "seeing" since it affects the ability of the telescope, 

 especially a large one, to focus sharply. Kesearch into seeing has 

 shown that these effects are most serious close to the land surface. 

 To minimize these effects telescopes are now located, at no small in- 

 convenience and expense, upon the summits of mountains in relatively 

 smooth air. In the best sites tlie average seeing diameter for a large 

 telescope is between 1 and 2 seconds of arc. Upon rare occasions the 



