CHAPTER XXV 

 ASTRONOMICAL PHOTOGRAPHY 



By Harlan True Stetson 



Fundamental Problems. — It is probably a fair estimate that 90 per cent of the 

 observational material gathered in the modern astronomical observatory is obtained 

 by means of photography. In a few specialized fields of astronomy, such as the 

 determination of fundamental positions of certain standard stars by means of the 

 meridian circle or transit, and more especially the physical observations of the surface 

 details of the planets or the measurement of the close double stars, the human eye 

 still seems to have the advantage over photography. Considerable experimenting, 

 however, has been done in recent years in applying the photographic plate and also 

 the photoelectric cell to instruments where times of transits of stars are desired. In 

 the determination of latitude, results obtained with a photographic zenith tube have 

 already shown an increase in precision over visual observations made with a zenith 

 telescope. 



The advantages of photography as applied to astronomy are twofold : 



1. A permanent and unbiased record is obtained reproducing the exact conditions 

 of any astronomical event for investigation or study, and for reference at any sub- 

 sequent date. Mistakes or errors in the measurement of the photograph may be 

 easily rechecked, but a mistake or error in the record of a visual observation can seldom 

 if ever be examined by an exact duplicate of the original observation. 



2. The effect of starlight on the photographic film is cumulative. The longer the 

 exposure, the greater is the amount of silver in the emulsion redixced. Experiments 

 with the human eye indicate that in about J-^ sec. or less the retina will record all 

 that the eye sees. Further exposure results in fatigue rather than enhancement of 

 vision. In the case of the photographic plate, however, prolonged exposure may 

 be used with given optical equipment to obtain images of celestial objects too faint 

 to be recorded with the telescope when the eye alone is used as the detector. 



Another advantage of the application of photography to the field of astronomy 

 is that the relative positions of a large number of stars representing the whole field 

 of the photographic plate can be recorded at one and the same time. For deter- 

 mination of position as well as the recording of the relative brightness of stars, the 

 photographic plate is a mass-production affair as compared with the highly individ- 

 ualized work of the visual observer where the entire attention must be given to one 

 celestial object at a time. 



Photography of celestial bodies entails certain unique problems, foremost of which 

 is some form of automatic mechanism that will keep the camera or photographic 

 telescope constantly directed to the object being photographed while the latter 

 apparently drifts across the sky as a result of the diurnal rotation of the earth on its 

 axis. No wholly automatic device has yet been invented that will succeed completelj' 

 in accomplishing the purpose, though experiments are now being conducted with this 

 end in mind. The human eye and hand are still necessary adjuncts in the control of 

 the direction finder if a star is to imprint an unblurred image on the photographic 

 plate during a prolonged exposure. 



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