ASTRONOMICAL PHOTOGRAPHY 



737 



The Double-slide Plateholder. — In the double-slide plateholder, no attempt is 

 made to introduce small compensating movements about the polar and declination 

 axes of the instrument, but the plateholder is moved slightly east and west or north 

 and south by means of micrometer screws provided in the mechanism to which the 

 plateholder itself is immediately attached. A small eyepiece or sometimes two eye- 

 pieces sliding in ways on the border of the 

 telescopic field and outside the dimensions of 

 the photographic plate itself may serve for 

 "finding" a suitable guiding star. The eye- 

 piece will contain the conventional cross wires 

 upon which a star image, obviously not the 

 one being photographed, may be located. In 

 the double-slide-plateholder method the main 

 lens or mirror which is used in photography 

 becomes also the objective for forming the 

 image of the guiding star. The cross wires 

 of the eyepiece in any guiding mechanism are 

 provided with some form of faint illumination, 

 so that the observer may see both the cross 

 wires and the image of the guiding star 

 simultaneously. The double-slide plateholder 

 has the advantage over the guiding telescope 

 or auxiliary finder in that in the case of the 

 large instruments the entire mass of the tele- 

 scope tube does not have to be moved to 

 follow the capricious waves of starlight intro- 

 duced through atmospheric disturbances. 

 While the photographic plate is being exposed, 

 the observer will keep his eye constantly on 

 the star in the guiding eyepiece and a hand 

 on each of two micrometer screws. A skilled 



observer may succeed in making several small movements with the micrometer 

 screws of this mechanism each second as he observes the guiding star, constantly 

 readjusting the plateholder to maintain the star at the intersection of the "wires." 



Lenses Used in Astronomical Photography. — The variety of problems encountered 

 in astronomy entail the employment of a wide variety of optical equipment. Con- 

 trary to popular opinion, much serious astronomical work not only can be done with 

 modest optical equipment but often can be performed more efficiently and with a 

 higher degree of satisfaction than with the use of large telescopes whose operations 

 are restricted to special problems. 



Bruce Doublet. — The introduction of the portrait lens into astronomy for the 

 photography of star fields covering several square degrees of the sky was largely due 

 to the genius of the late Edward Emerson Barnard, for many years astronomer at the 

 Yerkes Observatory. Barnard's early training involved an apprenticeship in a 

 commercial photography studio. Early in his career of astronomy he experimented 

 with portrait lenses attached to conventional telescope mountings. The excellent 

 results obtained in photographing sections of the Milky Way led to a specially 

 designed doublet with a unique mounting for photographing a complete map of the 

 Milky Way. The resulting instrument, named for the donor who contributed funds 

 to the Yerkes Observatory for its construction, became known as the Bruce telescope. 



It has become a standard pattern for an astronomical photographic doublet for 

 many years. The design of the Bruce telescope comprised actually three telescopes 



Fig. 1. — The double-slide plate- 

 holder designed by the author for the 

 Perkins Observatory. Here small, 

 specially constructed electric motors 

 provide for moving the micrometer 

 screws by remote control. 



