SATELLITE-TRACKING PROGRAM — ^HAYES 297 



THE NUNN CAMERA 



Joseph Nunn continued to -work on the design of the camera. By 

 September 1956 he had prepared a series of pictorial drawings that 

 indicated the general construction, and by late fall he had prepared 

 the first blueprints indicating the details, including the optical 

 features. 



Here it will be best to describe the camera as it is now used at the 

 photographic tracking stations. 



The camera must follow the path of the satellite as it moves across 

 the sky. A special mount is required for this purpose. 



Like a star, a satellite rises into the sky from the horizon, culmi- 

 nates, and then sets. Here the similarity in their paths ends. Satel- 

 lite culmination is not limited to the observer's meridian. Further- 

 more, its path on the celestial sphere is not necessarily restricted to 

 a semicircle, nor is it symmetrical about culmination. Consequently, 

 the angular velocity of the satellite as observed from a point on the 

 surface of the earth may change greatly between horizon and 

 culmination. 



For these reasons the traditional telescope mount that is designed 

 to track stars would be most inadequate to track satellites. On the 

 other hand, a mount that would follow exactly every possible path of 

 the satellite would be so complex as to be wholly misuitable in the 

 field. The mount designed is a simple yet effective compromise be- 

 tween these two extremes. 



Set in a gimbal ring, the camera can be turned on its triaxial 

 mounting at predetermined speeds to match the predicted motion of 

 a satellite. This speed can be continuously varied from zero to 7,000 

 seconds of arc per second of time. This latter speed is equivalent 

 to traversing the sky from horizon to horizon in 93 seconds. 



The gimbal ring and the drive mechanism are fitted into the yoke 

 of an orthodox altitude-azimuth mount. By making the necessary 

 settings for azimuth, altitude, and track angle, the operator can direct 

 the camera to any point in the sky above 15° elevation in such a way 

 that the camera will be driven through that point in the same direction 

 as that taken by the satellite during its passage. The actual point 

 for tracking is usually about culmination, the highest elevation of 

 the satellite as seen by the observer. 



The camera photographs on a 55-nun. film. The field of view of 

 the camera is 30° along the track of the satellite, and 5° perpendicular 

 to that track. The camera photographs at rates ranging from one 

 frame every 2 seconds to one frame every 32 seconds. 



The path swept out by the camera is a great circular arc 130° long. 

 In regions away from culmination, there is necessarily a divergence 



