250 BELL SYSTEM TECHNICAL JOURNAL 



the corona is separated from the glare by electrical filters while the 

 image of the sky around the sun is temporarily represented by an 

 electric current. 



Apparatus and Technique 



Dr. G. W. Cook, Director of the Cook Observatory, kindly offered 

 the use of his 15-inch horizontal telescope for a trial of the method. 

 Special television apparatus was developed at the Bell Telephone 

 Laboratories for use in conjunction with this telescope. This appa- 

 ratus has been called the coronaviser. Figure 1 shows a layout of the 

 complete apparatus. Starting at the left, the plane mirror M is cou- 

 pled to driving mechanism to form a siderostat so that sunlight may 

 be continuously held on the axis of the telescope. The solar disc, 

 about 2" in diameter, is focussed by the objective of approximately 

 18 feet focal length to fall on the tilted mirror which reflects the direct 

 sunlight out through a hole in the side of the telescope and into a light 

 trap consisting of a black walled tube with a black velvet end. Figure 

 2 shows the scanning mechanism in greater detail. Immediately in 

 back of the mirror R, which is the one just referred to for throwing 

 sunlight out of the telescope, a black masking disc D further prevents 

 sunlight from getting into the scanning apparatus. Several different 

 sizes of this disc are used to take care of the different diameters of the 

 solar image which occur throughout the year and to shield the scanner 

 from sunlight which spills over with bad seeing. This mirror and 

 disc are supported by means of the plate glass P so that there is no 

 obstruction in the field around the sun. 



To the right of the plate glass lies the scanning apparatus. This 

 is a mechanical device which scans the region of the sky around the 

 sun in a spiral, path. The scanning motion thus consists of a circular 

 and a radial motion. 



The simple plano-convex lens L which is silvered on the back is 

 equivalent to a concave mirror and forms an image of a portion of 

 the sky image that lies in the plane of D on the scanning hole H which 

 is on the axis of the telescope and scanner. The light that enters the 

 scanning hole passes through the lens, the prism, and the light tunnel 

 U into the photo-cell E. When the lens L is rotated about the axis 

 by the motor, the effect is the same as moving the scanning hole around 

 in the image plane at D, This takes care of the circular component 

 of the scanning motion. 



The radial component is obtained by changing the angle of tilt of 

 the lens L while it is rotating. A worm W is mounted on the motor 

 shaft but held stationary so that as the gear G revolves as a whole 



