ASTRONOMICAL PHOTOGRAPHY 735 



astronomical technology the word "doublet," however, is not generally used for such a 

 single combination of a pair of lenses. The word "doublet" is reserved for a combi- 

 nation of two achromatic lenses spaced some distance apart, as in the older type of 

 portrait lens, thus giving a relatively large field for the focal length. In the con- 

 ventional type of telescope constructed for visual observations of celestial objects, 

 the lens is made achromatic for wavelengths in the neighborhood of the D lines of 

 sodium, which is the part of the spectrum to which the eye is most sensitive. If the 

 telescope lens is specially made for photographic work, it is achromatized for a region 

 in the neighborhood of 4500 A. The telescope with such an objective, however, is 

 practically useless for visual observations since the color dispersion in the region of 

 5900 A is too great to produce a sharp focus. The so-called "visual lens" maj' be 

 utilized for photography, however, if a color "screen" or suitable filter is employed. 

 As a filter should be selected which will best transmit the flat part of the color curve 

 of the telescope lens, an orthochromatic plate or one that is yellow sensitive must be 

 employed since the yellow region of the spectrum alone will be available for photo- 

 graphic action. 



The Thaw telescope at the Allegheny Observatory is equipped with an achromatic 

 objective specifically designed for photographic light and has proved very effective for 

 astronomical photography where a great focal length is desired, as in determination of 

 star positions for "parallax" and "proper motion." 



In the reflecting type of telescope a mirror is employed which forms the image of 

 the celestial object by reflected light, the curvature of the mirror being figured in the 

 form of a parabaloid so that parallel light will be brought to a single focus. The 

 reflecting telescope or the reflector has the distinct advantage in astronomical photog- 

 raphy of providing a perfect achromatic optical system. Since the image is formed 

 by a reflection from a silvered or aluminized surface and not by light transmitted 

 through glass, no dispersion of light results. Another great advantage of the reflector 

 is that it is possible to construct a reflecting surface of far greater diameter than is the 

 case when clear glass disks must be provided for a system dependent upon transmitted 

 light. 



The present practice of depositing a reflecting surface of aluminum by evaporation 

 in a vacuum chamber in place of the chemical deposition of silver has brought about 

 marked improvement in the use of the reflector in astronomy. The aluminum coating 

 not only has the advantage of greater resistance to tarnish on exposure to air but at 

 the same time has a higher coefficient of reflectivity at the violet end of the spectrum , 

 to which the ordinary photographic plate is most sensitive. 



It is of astronomical interest to note that the reflecting telescope invented by Sir 

 Isaac Newton came into being as the antidote for the dispersional lens system. With 

 the development of the achromatic lens, refractors rivaled reflectors since the question 

 of the deterioration of the reflecting surface did not enter. In the latter part of the 

 nineteenth century when photography seriously began to replace visual observations, 

 the merits of the reflector were rediscovered, and our largest astronomical instruments 

 today are of the reflecting type. The 100-in. Hooker telescope of the Mount Wilson 

 Observatory of the Carnegie Institution of Washington is, and will probablj^ remain, 

 the largest telescope in the world until the completion of the 200-in. reflector provided 

 for Mount Palomar. 



One serious disadvantage of the reflector is the comparatively limited region of the 

 field that is sufficiently flat to be utilized in astronomical study. The adjunct of a 

 1 : 1 power lens system to flatten the field of the reflector is a recent development to 

 offset this difficulty. Since in astronomical photography one is usually concerned 

 with the obtaining of records of the faintest objects that may be recorded, the large 

 light-gathering power of a great reflector, together with its perfect achromatism. 



