SOLAR PROMINENCES IN MOTION — McMATH 123 



and had obtained successful motion pictures of solar prominences 

 late in 1932. Lyot, a Frenchman, invented and built his corono- 

 graph in 1930. His first application of his new instrument to 

 motion-picture photography of solar prominences was in September 

 1935. And now, a brief description of the solar research at the 

 McMath-Hulbert Observatory may be of interest. 



The rotation of the sun as shown by sunspots, the drift of sun- 

 spots and their changes, the more active type of solar prominences, 

 showing changes of form, were included in the original work pro- 

 gram of the observatory, and a specially designed spectroheliokine- 

 matograph for use in motion-picture photography of prominences 

 was constructed in 1931. This spectroheliokinematogi'aph had to be 

 light enough and compact enough to fit on the eyepiece end of a 

 101/2-inch equatorial Cassegrain reflecting telescope, but in spite of 

 these severe restrictions and the relative rarity of active prominences 

 during the sunspot minimum, prominence motions of sufficient in- 

 terest and complexity to warrant installation of more powerful and 

 versatile equipment for their study were photographed. A tower 

 telescope with pit spectrograph best suited the specifications that we 

 evolved from a study of the prominence motions from the spectro- 

 graph films and the then existing instrumentation of the leading 

 solar-research observatories. Accordingly, a tower telescope, em- 

 bodying our experience in astronomical motion-picture photography 

 and the adaptable features of other tower telescopes, together with 

 innovations leading to ease of control and manipulation, was erected 

 during 1935 and produced its first prominence pictures on July 1, 

 1936. 



With the completion of the new tower, the difficulties encountered 

 with the spectroheliokinematograph have been completely eliminated. 

 The observer may take pictures as rapidly as required, limited only 

 by a minimum effective exposure time of one-fiftieth of a second. 

 The effective wave length of the light producing the picture is under 

 complete control of the observer and may be varied throughout a 

 spectral range of 7,000 angstroms, and any setting may be dupli- 

 cated to a small fraction of an angstrom. The scale of the picture 

 may be changed in a short time, and the start and end of each ex- 

 posure are recorded automatically. With the usual type of spectro- 

 heliograph, perhaps 150 photographs could be obtained in an 8-hour 

 day; with the McMath-Hulbert tower, 15,000 photographs can be 

 obtained in the same observing time. Although the mechanical fea- 

 tures of observing are readily controllable and in a largo part auto- 

 matic, the observer still needs to exercise considerable skill in 

 avoiding too many pictures of a slowly moving object and too few 

 of objects in rapid motion. For the most part, prominence motions 



