ASTRONOMICAL PHOTOGRAPHY 757 



raphy of the sun, moon, and planets some special methods of exposure and procedure 

 are necessa^3^ Likewise, the photography of comets and of meteors requires special 

 technique. The application of photography on occasions of total solar eclipses pre- 

 sents again unique problems deserving special consideration. 



Photography of the Sun. — Photographs of the sun may be taken satisfactorily with 

 almost any telescope, provided the light is sufficiently diminished and the exposure 

 sufficiently short. In solar photography it is desirable to use a diaphragm over the 

 objective with an aperture which will provide a focal ratio in the neighborhood of from 

 //50 to //1 00, the smaller ratio being used in the case of telescopes of large aperture. 

 For making the exposure, a focal-plane shutter capable of rendering an exposure of 

 Kooo sec. is desirable. Care must be taken in case of larger focal ratios that a black 

 curtain shutter is not unduly exposed to the image of the sun, as it may be burned 

 through the excessive heat. For this reason use is sometimes made of a metal shutter 

 carrying a narrow slit that may be passed rapidly in front of the plate. The author 

 has found a very serviceable solar camera can be made by utilizing the back of an 

 old Graflex. The back must be provided with a suitable adapter for attaching to the 

 eye end of the telescope. The curtain shutter is set for time exposure with the full 

 aperture open to the ground-glass screen for focusing the image of the sun; the plate- 

 holder is then inserted, the curtain screen reset for exposure, the slide drawn, and the 

 exposure quickly made. As soon as the exposure is made, the telescope is directed 

 away from the sun. In this mode of operation there is little danger from the heat on 

 the curtain shutter. In the case of a visual refractor a color filter must, of course, be 

 employed. 



In professional equipment for solar photography at observatories specializing in 

 this work, the telescope may be permanently fixed in a horizontal or vertical position 

 and light from the sun fed to the objective from a heliostat or coelostat. In the case 

 of the heliotelescope the entire building housing the lens and plate forms in itself a 

 darkroom and replaces the more conventional telescope tube. A coelostat is placed in 

 the open under separate shelter, light from which passes through a circular window in 

 the observing house to the telescope objective. 



At the Mount Wilson Observatory^ near Pasadena, Calif., and the McMath-Hulbert 

 Observatory at Lake Angelus, Mich., a vertical form of telescope is employed for solar 

 work. The telescope tube is held in a vertical position by suitable supports to a 

 surrounding tower. At the top of the tower is the coelostat, easily protected from the 

 weather by a dome. The movements of the dome and of the coelostat itself 

 are effected by the use of motors with remote control at the ground end. An observing 

 shelter completely enclosed is constructed at the ground end of the telescope tube. 

 The solar image is formed directly on the plate which reposes in a horizontal position. 

 Because of the form of structure employed in the vertical telescope, such an instrument 

 is frequently called a "tower telescope" or "solar tower." 



A tower telescope has an advantage over the horizontal telescope in that the atmos- 

 pheric disturbances in the case of the vertical tube are less disastrous than is the case 

 in the horizontal telescope where heated strata of air near the ground are constantly 

 rising across the optical path. Since at best, in photographing the sun, a large amount 

 of heat from the ground and surrounding objects creates a turbulence of air through 

 which the light waves pass, it has been found in some localities that the best hours of 

 the daj' for solar photography are in the earlj^ morning before the air and ground have 

 become highly heated. 



Because of the value of a knowledge of the number and distribution of spots on the 

 surface of the sun, a group of observatories throughout the world are now cooperating 

 in securing daily photographs of the sun, the results of which are forwarded in this 

 country to the U. S. Naval Observatory in Washington which publishes monthly a 

 summary of solar data. 



