SATELLITE -TRACKING PROGRAM — ^HAYES 293 



THE BAKER OPTICS 



During the spring months of 1956 Dr. Baker made further calcula- 

 tions on modified Schmidt systems. Such a system might, for example, 

 consist of a cemented doublet correcting plate and an aspheric mirror. 

 It became quite clear, however, that the difficulties of the problem 

 had increased faster than simple modifications of the classical Schmidt 

 could accommodate. Dr. Baker brought this situation to Dr. Wliip- 

 ple's attention and requested financial support to conduct a searching 

 examination of more involved optical systems. 



Authorization to proceed was obtained in June. Dr. Baker then 

 began detailed calculations of systems of increasing complexity to 

 find the simplest that would meet the new requirements. Before 

 arriving at the final solution, he analyzed exhaustively three other 

 simpler systems and various intermediate systems, and rejected them 

 all." 



Nevertheless, with the completion of calculation for the third 

 system, Dr. Baker knew that he had "cracked" the problem. It was 

 now largely a question of finding more suitable glass types. The 

 design of the corrector cell itself represented a compromise; if the 

 air spaces between the lenses of the cell are too large, the aspheric 

 powers, even though weak, lead to unacceptable astigmatism in the 

 outer field. If the air spaces are too small, the aspheric powers be- 

 come too great for practical manufacture. Therefore, in applying the 

 concept of the three-lens corrector cell, he had to interpolate more or 

 less along a curve to reach a point that represented as small an air space 

 as would be practicable for a system that could actually be manufac- 

 tured. A larger air space than was decided upon would have simplified 

 the manufacturing problem, but the astigmatism in the outer field 

 would have made for unhappy results well outside the 20-micron 



w The first of these, already mentioned, was an achromatized classical Schmidt system 

 modified to include an aspheric primary mirror. The second system made use of two air- 

 spaced corrector elements and an aspheric mirror. The performance of this second sys- 

 tem was appreciably superior to that of the standard classical Schmidt but otherwise was 

 discouraging because of residual higher-order coma, or "halo" around the image. He 

 was forced to go to a more complicated system. Drawing on more than 20 years' ex- 

 perience, and choosing ordinary types of optical glass selected after much computing, he 

 designed his third optical system, which met the monochromatic requirements ; its per- 

 formance was, in fact, more or less identical to that of the present satellite camera in 

 the optimum region of the spectrum. The chromatic aberrations, or color blur, of this 

 combination of ordinary glass types turned out. however, to be well beyond the assigned 

 tolerances. At the extremes of the spectral range, the image formed by this system 

 would have been 5 times larger in diameter and 25 times larger in area than the specified 

 20-micron spot size. The monochromatic characteristics of the system did prove the value 

 of the three-plate corrector cell. It would not, however, have been at all safe to adopt 

 the design, inasmuch as the effect of the color blur would have been rendered all the more 

 disastrous because the aperture of the camera would be necessarily much obscured by 

 the presence of film and shutter in the light beam. These shadowing effects would have 

 been present in the enlarged star images and would have made precision measurement 

 most difficult. 



