294 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



tolerance. Clearly, this same curve was also a curve of dollars, and 

 it was necessary to be most careful to strike an acceptable balance 

 between cost and performance. 



The choice of suitable glasses for the corrector cell necessitated 

 finding a combination of types that would remove or satisfactorily 

 reduce the so-called secondary spectrum that had been the remaining 

 dominant aberration of system No. 3. The catalog of Schott, the well- 

 known German manufacturer of optical glass, listed only a few types 

 providing negative dispersive characteristics of the proper trend 

 throughout the spectrum. Of these only KzFS-2 seemed to be at 

 all acceptable. Although listed as greenish in coloration, it was 

 considerably whiter than the KzF series and, if kept thin, would 

 provide adequate transmission in the ultraviolet. Unfortunately, 

 this glass was also described as sensitive to acid staining and "soluble" 

 in distilled water, though to a less degree than KzFS-3. Nevertheless, 

 there was no way to solve the optical problem except to adopt 

 KzFS-2 for the outermost pair of elements in the correcting trio of 

 elements. The sensitivity of KzFS-2 to moisture and to atmospheric 

 acid staining has continued to plague operators of the Baker-Nunn 

 cameras, yet by no means to the degree earlier anticipated. Although 

 the fact is perhaps not obvious, the transmitted light actually used 

 in taking satellite pictures is far less affected by the staining of 

 the KzFS-2 surfaces than the reflected light, not otherwise used, 

 would indicate. 



The central element of the corrector cell had to be matched to 

 KzFS-2. Dr. Baker finally chose SK-14, a glass of fairly high index 

 compared to KzFS-2 but known to have very good transmission. 



An important but unlooked-for bonus in the choice of these two 

 glasses was that they have very little thermal expansion, at least com- 

 pared with that of many other optical glasses. In this respect they 

 were unusually well matched, a fact that simplified the design of the 

 means by which the elements of the corrector cell would be held in 

 accurate alignment. 



This three-element corrector cell as finally designed would provide, 

 then, an image with a minimum of color aberration and a minimum of 

 coma. It was now matched to a mirror 30 inches in diameter, the 

 best glass for which was Pyrex, made by the Corning Glass Works in 

 Corning, N.Y. 



Dr. Baker completed the optical design of the camera by the end 

 of July 1956. Already it was a much more complicated system than 

 either he or the staff of the Observatory had ever anticipated. As the 

 months went by, it proved to be considerably more expensive to manu- 

 facture and involved a larger, heavier camera than any of them had 

 originally thought. 



