THE USE OF A PHOTOGRAPHIC DOUBLET IN CATA- 

 LOGUING THE POSITIONS OF STARS. 



By frank SCHLESINGER. 

 (Read April 24, 1914.) 



In order to utilize for cataloguing and for similar purposes the 

 positions of stars derived from photographs, it is necessary to know 

 the scale of the plates, their orientation and their positions in the 

 sky. In the early days of astronomical photography attempts were 

 made to determine the scale and the orientation by such methods as 

 measuring the absolute focal length of the photographing tele- 

 scope, and by impressing an orienting star trail upon the plate. 

 These attempts have not been successful and it has now become the 

 universal practice to employ comparison stars for these purposes : 

 that is, stars that appear on the plate with positions known in ad- 

 vance, usually through observations with the meridian circle. Here 

 again experience has shown that in the determination of star places 

 by photography, this matter of comparison stars is usually the 

 weakest link in the chain. For example, in the case of the Astro- 

 graphic Catalogue much of the relatively high precision with which 

 the plates can be measured is lost on account of the lack of suitable 

 comparison stars. 



An experiment that aims to overcome this difficulty is in progress 

 at the Allegheny Observatory. Instead of employing a simple 

 objective to photograph the stars, a doublet lens is used. This has 

 the advantage of much greater extent of field of good definition, at 

 least six times as great as in the case of the objectives used for the 

 Astrographic Catalogue. • Consequently in surveying any large area 

 of the sky, plates taken with the latter kind of telescope will require 

 (other things being equal) six times as many comparison stars as 

 the doublet. 



The use of the doublet for these purposes was advocated twenty- 

 five years ago by Pickering, but astronomers have feared that the 

 use of such an objective might introduce serious errors of various 

 kinds. The experiments already completed at Allegheny prove that, 



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