744 HANDBOOK OF PHOTOGRAPHY >■ 



motor moves the main mirror at a rate of one complete rotation about the polar axis in 

 48 hr., not once in 24 hr. as is the case of the directly mounted telescope. The explana- 

 tion is, of course, that the motion of the beam of light reflected from a moving surface 

 is displaced by twice the angle at which the reflecting surface is displaced in the unit of 

 time. 



Photographic Plates and Stellar Spectra. — In commercial photography at the 

 present time, photographic emulsions are largely used on celluloid films. The celluloid 

 film has the advantage of lightness in weight, small storage space, and freedom from 

 breakage. In astronomical work, glass, however, is still extensively used for support- 

 ing the photographic emulsion. The chief reason for this is the need for making 

 accurate measurements between positions of stars recorded on the plate and for deter- 

 mining with high precision the wavelengths of lines in stellar spectra. For qualitative 

 work to some extent, celluloid film may be employed for astronomical purposes where 

 the question of expansion or shrinkage of the emulsion with moisture or temperature 

 is not sufficientlj' serious to promote observable distortion. One good example of the 

 use of celluloid film in astronomy is in connection with motion-picture photography 

 of solar prominences and flocculi with the spectroheliokinematograph described in an 

 earlier section. 



As much of the observing, however, is astrometrical in nature, the celestial photog- 

 rapher must make provision for adequate storage of glass plates. As each glass plate 

 is essentially an astronomical record of some portion of the sky at a given instant, 

 a collection of such plates forms a veritable photographic library to which references 

 may be later made in case of the appearance of new stars or the discovery of comets, 

 meteors, or minor planets whose earlier history may have escaped attention at the 

 time of the original exposure. The most extensive photographic library of the sky 

 exists at the Harvard College Observatory which now comprises some 400,000 plates. 



Speed vs. Grain. — There are two important requisites in the selection of plates for 

 astronomical purposes: (1) speed and (2) grain. Since exposures upon faint celestial 

 objects are necessarily long, often entailing the entire period of darkness in a single 

 night, it is desirable that the emulsion selected should be as fast as is possibly consistent 

 with quality and with size of grain. Some rapid plates that have been developed have 

 poor keeping qualities and contain man}^ blemishes, which, while not a serious handi- 

 cap in commercial photography, render the plates prohibitive for scientific use, since 

 blemishes and unequal sensitivity may cause false deductions when astronomical 

 objects are examined. In general a fast plate is of coarse grain impeding the detailed 

 recording of celestial objects requiring the highest possible resolution. Furthermore, 

 since stellar images are at best but a small aggregation of reduced silver grains and 

 since the distances between many pairs of images on a photographic plate are often 

 to be measured with the micrometer microscope, a plate of coarse grain introduces 

 difficulties for the measurer in endeavoring to bisect the image of the star with the 

 spider thread of a micrometer microscope. For many purposes, therefore, a relatively 

 slow plate of fine grain is preferable, even though it may add materially to the patience 

 required of the observer by increasing the exposure time at the telescope. 



The question of the building up of the photographic image imder the action of light 

 and the law of increasing density with illumination and exposure time, commonly 

 known as the "reciprocity law," have been adequately described elsewhere in this 

 handbook. The response of different emulsions to light for a given exposure depends 

 not only on intensity and exposure time but quite as much upon the spectral distribu- 

 tion by wavelengths. The emulsions, therefore, must be selected with a view to color 

 response to which they are most sensitive. The accompanying chart represents the 

 wavelength response of some typical emulsions produced by the Eastman Kodak Co. 

 for astronomical and spectroscopic work. At the lower end of the diagram it will be 



