The Coronaviser, an Instrument for Observing the Solar 

 Corona in Full Sunlight * 



By A. M. SKELLETT 



Introduction 



BECAUSE of the rarity of solar eclipses, their short duration, and 

 their occurrence usually at inconvenient places on the earth's 

 surface, the problem of observing the solar corona in full sunlight is 

 an important one for astronomers. It is also of considerable interest 

 to those telephone engineers who are concerned with radio transmission 

 over long distances. The major disturbances of such transmission 

 have their origin in the sun and studies to date have indicated that a 

 day-to-day knowledge of the activity of the corona might prove useful 

 in predicting the transmission conditions. 



The first attempt to solve this problem was made by Huggins in 

 1878 and since that time every conceivable optical means to accom- 

 plish the desired result has been tried. The problem is to observe 

 the corona, not in itself a faint object, through the blinding glare of 

 the sky in the region around the sun. If one holds his hand at arm's 

 length so that it blots out the sun, he will find the glare in the sky 

 around it so intense as to be painful. It is generally at least a thousand 

 times brighter than the corona. The trials have usually been made 

 at very high altitudes where the atmospheric glare is greatly reduced 

 but since the scattered light from the telescope itself, particularly the 

 objective, is some hundreds of times brighter than the corona no suc- 

 cess was obtained until M. Lyot ^ invented his coronographe, a tele- 

 scope in which this latter kind of glare is greatly reduced. With this 

 instrument at the top of Mt. Pic du Midi in the Pyrenees mountains 

 he has obtained several photographs which show some of the features 

 of the inner corona. At best he has to work with a glare that is nearly 

 as bright as the brightest parts of the corona. There are only a few 

 days in the year when the intensity of the glare is low enough to enable 

 him to observe coronal features through it. 



It is obvious that a method of greater discrimination is needed if 

 day-to-day observations are to be made. Such a method was pro- 

 posed several years ago.^ It is based on the use of television technique; 



* Presented at mtg. of Nat. Acad. Sciences, Providence, R. I., October 1939, and 

 before Amer. Philos. Soc. in Philadelphia, Pa., November 1939. 

 1 Lvot, B., M. N. R. A. S., 99, 8, 580, 1939. 

 ^Skellett, A. M., Proc. Nat'l. Acad. Sci., 20, 461, 1934. 



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