ASTRONOMICAL PHOTOGRAPHY 743 



University of Michigan. Such a device is called a "spectroheliokinematograph." 

 At the present date (1939) a new tower telescope with spectroheliographic and spectro- 

 heliokinematographic equipment has been constructed at the site of the McMath- 

 Hulbert Observatory at Lake Angelus, Mich. By means of motor-driven mirrors, 

 sunlight is brought through the top of the tower vertically downward to the slit of 

 the spectrograph in the observing room. The spectrograph itself is contained in a well 

 extending below ground to a depth of 31 ft., thus insuring convenient temperature 

 control. A 6-in. grating of 15,240 lines to the inch is used for the formation of the 

 spectrum. The motion-picture camera is a modification of the Bell and Howell Super- 

 speed check-pawl mechanism such as has been made only on order for the large picture 

 producers. Synchronous motors insure uniform motion, both for the film and for the 

 motion of the slit with respect to the image of the sun. An excellent brochure on 

 the design and detailed construction of the equipment is contained in Publications of the 

 Observatory of the University of Michigan, Vol. 7. The most remarkable motion pic- 

 tures of solar eruptions ever yet recorded have already resulted from the ingenuity and 

 skill of the McMath-Hulbert observers. From the point of view of science, this is 

 perhaps one of the most remarkable adaptations of photography of modern times. 



In addition to the photography of the solar prominences and flocculi with the 

 spectroheliokinematograph, mention should be made of the very successful work of the 

 McMath-Hulbert Observatory in producing motion pictures of the moon showing its 

 rotation, the rising and setting of the sun, and conspicuous lunar craters and moun- 

 tains. Motion-picture photography of this observatory has also included probably 

 the finest record of a total eclipse of the sun yet recorded on motion-picture film. 



Heliostat, Siderostat, and Coelostat.— In many problems of astronomical photog- 

 raphy involving heavy apparatus of a complex nature such as a spectrograph or a 

 spectroheliograph, it is often desirable to have the telescope fixed in either a horizontal 

 or vertical direction. Under such circumstances, an auxiliary mirror driven by clock- 

 work, or a synchronous motor, reflects light from the celestial objects into the lens 

 system. One type of instrument used for reflecting light from the sun or star in a 

 constant direction is called a "heliostat" or sometimes a "siderostat." This consists 

 of an optical-plane mirror mounted on a polar axis turned by the driving mechanisms. 

 The mirror is also pivoted along a diameter at right angles to the polar axis, so that it 

 may have a supplementary motion in declination to reach the sun or star on either 

 side of the celestial equator. A great disadvantage of this single mirror heliostat is 

 that the field of view rotates in the telescope as the mirror is driven bj' clockwork 

 about the polar axis. To obviate this difficulty, the coelostat has been devised. In 

 the coelostat a second mirror is placed south of and above the level of the primary 

 mirror from which the beam of light is first reflected. By the use of this second mirror 

 the light beam received from the moving mirror can be sent in any convenient direc- 

 tion, irrespective of the position of the sun or the celestial object. However, on 

 account of the changing declination of the sun and the wide difference in declination 

 of the star, it is necessary to mount the secondary mirror on a carriage capable of longi- 

 tudinal motion in two directions. Or, as is sometimes the case, the primary mirror of 

 the heliostat may be mounted on the carriage providing longitudinal motion east and 

 west, while the secondary mirror may be moved along a track north and south. The 

 adjustment of the carriages on the track provide for reaching light from the sun or 

 stars in almost any position of the sky and directing it along the axis of the telescope 

 to be placed in either a vertical or horizontal position. 



This combination of two mirrors, which comprises the coelostat, has the advantage 

 that the field does not rotate with the turning of the main mirror about the polar axis 

 if the latter has been adequately adjusted parallel to the axis of the earth for the station 

 of observation. It may be remembered that in the case of the coelostat, a clock or a 



