738 



HANDBOOK OF PHOTOGRAPHY 



on a single mounting — two photographic objectives, one of 10 in. in diameter and the 

 other of 6 in. in diameter, and a visual telescope, of appropriate focal length. Each of 

 the photographic telescopes had as its lens system two pairs of achromatic lenses 

 separated by an appropriate distance. The relatively large ratio of aperture to focal 

 length, //5, made the equipment particularly fast. The photographs of star clouds 



and comets and the exquisite atlas of the Milky 

 Way which were obtained as the result of many 

 years' work won for Barnard the medal of the 

 Royal Astronomical Society of Great Britain. 

 Success with the Bruce doublet resulted in the 

 introduction of similar equipment in many other 

 observatories. 



The short-focus doublet in a variety of sizes 

 has proved an invaluable tool for mapping the 

 entire sky. The "Harvard Sky" maps cover 

 the entire heavens on 55 plates showing stars to 

 about the twelfth magnitude. Another extensive 

 star map is comprised in the Franklin Adams 

 charts which cover the entire heavens to stars of 

 about the sixteenth magnitude. The Franklin 

 Adams charts were made with cameras of 6- and 

 10-in. apertures and yield a scale of approxi- 

 mately 1° = 15 mm. 



An astrographic chart of the entire heavens 

 together with a catalogue giving the positions of 

 stars measured from photographic plates was 

 undertaken about 1900 with the cooperation of 

 observatories in all parts of the world. The 

 project is under the auspices of the International 

 Astronomical Union and has been about half 

 completed. The instrumental equipment of the 

 observatories cooperating has been standardized. 

 The objectives of the telescopes have an aperture of 344 mm. and a focal length of 

 3.44 m. Each plate covers an area of the sky approximately 2° square. As the lenses 

 of the astrographic telescopes, however, comprise only two-element objectives, the 

 2° square field shows rather poor images at the peripherjr. 



Choice of Aperture. — The choice of aperture of a photographic telescope will depend 

 upon the length of time of the exposure that one may profitably allot to obtain the 

 registration of star images of a given degree of brightness. The scale of brightness of 

 stars in common use in astronomy is designated as a "scale of magnitude." Stars 

 just visible to the naked eye fall generally in the classification of sixth magnitude stars. 

 Stars of the fifth magnitude are approximately 2.5 times brighter, whereas a star of the 

 seventh magnitude is 2.5 times fainter than one of the sixth magnitude. The law- 

 representing the ratio in brightness of a star of a given magnitude Mi to that of 

 magnitude M^ is given by the following: 



Fig. 2. — Ten-in. Bruce tele- 

 scope, designed and constructed 

 by the Warner Swasey Company, 

 Cleveland. 



log ^ = 0.4(M2 



02 



Ml) 



(1) 



The limiting magnitude found on star charts of the Franklin Adams series is about 

 16. With prolonged exposure of the largest telescope equipment, it has been possible 

 to photograph stars to about the twenty-first magnitude. Generally a photographic 



