STORMY WEATHER ON THE SUN — W. 0. ROBERTS 



171 



cover 



guider lens 

 diaphragm A 



photoelectric 

 guider 



scale in feet 



Figure 1. — The operating principles of the coronagraph are illustrated in this schematic 

 diagram of the High Altitude Observatory coronagraph. Lyot, inventor of the corona- 

 graph, found that sunlight scattered into the image of an ordinary telescope from the 

 following sources rendered such telescopes useless for artificial eclipse photography: 



(1) Diffraction of light from the edges of the objective lens cell. 



(2) Reflection from the second face of the objective lens to the first, and then a second 



reflection back into the instrument. 



(3) Diffraction around dust particles and imperfections of the objective lens. 



Any one of the three sources is enough, usually, to render an ordinary telescope useless 

 for coronal photography. The first two may be eliminated completely and simply. The 

 third may be reduced by employing the very highest skill in making the telescope objective 

 and in cleaning it. Diffraction at the edges of the objective lens can be eliminated by 

 placing a diaphragm (A) with a sharp edge in front of the objective, and by means of the 

 field lens focusing an image of this diaphragm (thus focusing the diffracted light) on 

 diaphragm B, just In front of the camera lens. If diaphragm B is slightly smaller than the 

 image of diaphragm A, all scattered light from the first source will be eliminated. The 

 multiple reflections from the objective form a small image of the sun very near the objective 

 lens. This small Image, since it lies near diaphragm A, will then be focused by the field 

 lens at a position near diaphragm B. If a small opaque screen is placed on the camera 

 lens, at Its center, the small solar Image formed by multiple reflections will be focused 

 upon it by the field lens, and thus prevented from reaching the 35-mm. camera film. 



In normal operation the coronagraph, thus, works as follows: The objective lens Images 

 the sun and its corona at the position of the occulting disk; at that point the light from 

 the face of the sun Is blocked by the occulting disk. The camera lens then Images the 

 artificially eclipsed sun on the 35-mm. camera film. In an alternative arrangement, a 

 mirror located beyond the field lens can be used to direct the image into the spectrograph. 



