734 HANDBOOK OF PHOTOGRAPHY 



Unlike terrestrial photography, the source of light from celestial objects is outside 

 the atmosphere of the earth and is bent or refracted upon entering it at an oblique 

 angle. Since the observer is constantly changing his position with respect to the 

 direction of a star on account of the motion of the earth, refraction is a variable 

 quantity and is particularly mischievous in dislocating the line of direction of objects 

 in the sky near the horizon. Were it possible, therefore, to effect a mechanical drive 

 which with unfailing uniformity would offset the motion of the earth, refraction alone 

 would necessitate continual "guiding" for a photographic plate with respect to the 

 star. Moreover, since the usual optical equipment for astronomical photography 

 employs an optical system of great focal length compared with the usual camera, 

 small irregular movements of star images are brought about through the turbulence 

 of the atmosphere through which we view them. 



It may be pertinent to remark that the so-called "image" of the star obtained on 

 the photographic plate is not in a strict sense an image of the star. All stars except 

 the sun are but luminous points of light at such great distances from us that no 

 telescope yet made or that we may hope to make could ever resolve their size or shape. 

 The so-called image obtained on the photographic plate is therefore but a cluster of 

 silver grains reduced by the feeble light falling in a restricted location on the emulsion. 

 In general, the center of this cluster of silver grains will correspond to the mean position 

 of the light beam producing the reduction. Furthermore, since the star is sensibly 

 a point source, a diffraction pattern results, and the photographic "image" of the 

 star becomes therefore the impression gained through the integration of the effect of 

 light diffracted through the circular aperture of the telescope. 



There are many problems peculiar to astronomical photography when we consider 

 the interpretation of the photographic record. If it is the position of the stellar images 

 with which we are concerned, a question of distortion of the film immediately enters 

 as a source of error. Furthermore, the degree of flatness of the field and its rectilinear 

 character are important factors in correcting for distortion of the scale on the plate, a 

 distortion which would depend upon the distance of the image from the optical center 

 or axis. Either of these problems enters when we come to the interpretation of the 

 amount of silver reduction as an indication of the relative light-giving power or magni- 

 tude of the star represented. While the technique of handling plates, making 

 exposures, or developing is much the same in astronomical photography as in any 

 other field to which photographic processes may be applied, certain factors such as 

 the effect of temperature and moisture and a variation in the sensitivity of the emulsion 

 become of serious consequence, when the plates are used for quantitative results, if 

 the exposure has been prolonged. These problems will receive detailed attention in 

 subsequent paragraphs. 



Instrumental Equipment. — With celestial photography the "camera" is generally 

 a telescope of recognized form in which the eyepiece, which would be used for visual 

 observations, is replaced by a convenient receptacle for carrying the plateholder that 

 contains the photographic emulsion. Telescopes may be divided into two major 

 classifications: (1) the refracting type in which an achromatic lens at the upper end 

 of the tube gathers the light and forms the image on the photographic plate; (2) the 

 reflecting type where a concave mirror at the lower end of the tube does the work of a 

 lens as the objective and brings the light to a focus on the plate at the upper end of the 

 tube. 



Refracting and Reflecting Telescopes. — The simplest practical form of telescope 

 lens is a doublet, consisting of a double convex element of cro^^Tl glass paired with a 

 concave lens of flint glass. In the more common form the crown lens is placed at 

 the upper end of the tube and is backed by the flint lens, whose negative curvature 

 may be made to conform to the curvature of the rear side of the crown disk. In 



