ASTRONOMICAL PHOTOGRAPHY 741 



length necessitates a removal of a large amount of glass near the center of the spherical 

 mirror, thus adding to the difficulties of grinding. Furthermore, a lens to introduce 

 the minimum amount of chromatic aberration entails a complicated figure which is 

 somewhat convergent in the center, becoming divergent toward the edge. The field of 

 the camera is of high curvature and requires the molding of plates or fUms to conform 

 with the curved field. In spite of these difficulties, no other camera has been con- 

 structed combining high speed with the wide field obtainable in the Schmidt 

 arrangement. 



Auxiliary Equipment. — A photographic plate is the logical medium for spectro- 

 scopic work in astronomy, hence various forms of spectrographs are employed in con- 

 nection with the telescope for the analysis of starlight. As the user of this handbook 

 is hardly likely to enter the highly specialized field of astronomical spectroscopy 

 unless he be professionally employed at an observatory, a general description of 

 spectroscopic instruments as an illustration of the adaptation of photography to 

 astrophysics will suffice. 



Stellar Spectrographs. — The simplest form of stellar spectrograph and one widely 

 used where small-scale spectra of large numbers of stars are required, such as for 

 general classification purposes, is the so-called "objective prism." The objective 

 prism as the term implies is a prism of glass, preferably of an ultraviolet-transmitting 

 quality, that is placed immediately in front of the objective of the telescope or astro- 

 nomical camera, thus dispersing starlight from each star in the field of the telescope. 



Since each star is essentially a point source of light, the image of the star photo- 

 graphed will be drawn out into lines with interruptions corresponding to the wave 

 lengths represented by the absorption frequencies of the stellar atmosphere. In order 

 to give an appreciable breadth to the spectra, the base of the prism may be oriented 

 parallel to one axis of the telescope and a slow motion imparted about the other axis, 

 thus drawing the slender spectrum into suitable breadth for examination purposes. 

 The interruptions in the slender spectra, therefore, broaden out into lines or bands 

 characteristic of the appearance of a spectrogram made with the ordinary laboratory 

 slit spectroscope. 



For telescopes of comparatively great focal length a relatively small angle, e.g., 

 5 to 10°, for the dispersion prism may be adopted and yet render a sufficient length of 

 spectrum for classification purposes. To take advantage of the full amount of the 

 beam of starlight falling on a telescope of given aperture, it is obviously necessary that 

 the aperture of the prism should be as large as that of the telescope. The great 

 advantage of the objective prism is that it uses the full amount of starlight available 

 and at the same time records large numbers of spectra on a single plate. 



When photographing stars with the objective prism, the telescope, of course, can- 

 not be pointed in the direction in which stars are seen, but allowance has to be made 

 for the angle of refraction through the particular prism employed. This entails some 

 slight inconvenience in directing the telescope to the sky as compared with the employ- 

 ment of the more conventional slit spectrograph where the telescope is directed 

 immediately to the star, the light of which falls on the slit as would be the case of any 

 light source utilized in laboratory apparatus. 



For a study of stellar spectra of larger dispersion and greater resolution than can 

 be photographed by the objective prism, the slit spectrograph must be employed. 

 This form of apparatus obviously is necessary for determining shifts in wavelengths 

 of stars introduced by velocity along the line of sight. Utilizing Doppler's principle, 

 radial velocities of approach and recession for many thousands of objects in the sky 

 have now been determined at the major observatories. A slit spectrograph for astro- 

 nomical work differs little in form from that of a standardized laboratory instrument, 

 except for the criterion of design necessitated by the optical system of the telescope 



