STRATOSPHERIC PHOTOGRAPHY — SCHWARZCHILD 327 



returned home in the fall of 1959 with a couple of hundred high- 

 definition solar photographs. These contained not only detailed pic- 

 tures of the granulation, both in undisturbed and in highly disturbed 

 magnetic regions, but also full-time sequences of both types of areas. 

 Thus it became possible in the subsequent analysis to determine not 

 only the distribution of sizes of convective elements in the solar atmos- 

 phere but also the average period of time a typical convective element 

 exists. These observational data have greatly strengthened our theo- 

 retical picture of convective heat transport in stars. As a matter of 

 fact we at Princeton as well as astronomers at other institutions are 

 continuing with the theoretical developments helped and stimulated 

 by these measurements. 



The sun is by no means the only celestial object of which higher 

 definition photographs are needed for the solution of fundamental 

 astronomical research problems. The sky is full of objects the essen- 

 tial details of which are blurred on photographs taken with telescopes 

 on the ground. There is Venus with its cloud cover, the structure 

 of which has hardly been glimpsed. There is the great Orion gas 

 nebula in which we are sure from indirect evidence stars are now 

 being formed; but whether this giant gas mass is smooth or knotty 

 or filamentary we still cannot judge from our present photographs 

 though we need to know before we can securely develop a theoiy of 

 the origin of stars. There is the Andromeda spiral nebula with its 

 incredibly dense stellar nucleus defying photographic resolution. 

 Many items can be added to this list, all referring to objects that are 

 typical examples of the celestial phenomena filling the universe around 

 us. Of all these it is only for the sun that the modest aperture of 12 

 inches of Stratoscope I would suffice to obtain substantially sharper 

 photographs than those already available from the ground. The other 

 objects would require a telescope with at least a 36-inch aperture. 

 After the first successful flights of Stratoscope I it was tempting to 

 start studying the feasibility of a larger balloon-borne telescope and 

 in due course we did begin the design and construction of such an 

 instrument — now called Stratoscope II. 



The requirements regarding optical perfection and pointing accura- 

 cy are, of course, much higher for the larger Stratoscope II than they 

 were for Stratoscope I. For example, the pointing accuracy will have 

 to be better than a thirtieth of a second of arc over exposure times as 

 long as 1 hour to make Stratoscope II fully effective. The require- 

 ments on optical perfection and on guidance are much less stringent 

 if Stratoscope II initially is used not for high-definition photography 

 but for spectrophotometric investigations in the infrared. The latter 

 presents another effective astronomical use of a balloon-borne tele- 

 scope since the few percent of the atmosphere above 80,000 feet are 

 practically transparent in the infrared (though they are still entirely 



