NATURE 



[December i, 1910 



THE PHOTOGRAPHY OF NEBULA. 



SINCE the year 1880, when Henry Draper, of New- 

 York, achieved the first success in pliotograplimg 

 nebula;, namely, the great nebula in the constellation 

 ot Orion, the progress made in this branch of astro- 

 nomy has been botli rapid and secure. In this country 

 Common and Isaac Roberts, in France Janssen and 

 the brothers Henry, in Uermany Max Wolf, and in 

 the United States W. H. Pickering, Barnard, and 

 Keeler, all have helped to obtain the high standard 

 of excellence which prevails to-day. 



Both refracting and reflecting telescopes have been 

 rivalling each other to obtain the mastery in this 

 particular branch, and I think that it is generally 

 conceded to-day that the latter have won the day. The 

 great success achieved is no doubt partly due to the 

 important progress made in the preparation of the 

 photographic dry plate, but a closer scrutiny of the 

 whole situation brings into the light the peculiar skill 

 of the man at the telescope. Isaac Roberts, for in- 

 stance, had not a very large reflector to work with, 

 one of only 20 inches aperture, yet his skill in tuning 

 up his instrument and his very careful "following" 

 were rewarded by the magnificent set of wonderful 

 photographs which he was able to secure. 



Again, Keeler, with the Crossley three-foot re- 

 flector, an instrument made in 1879 by Dr. Common, 

 which only reached the United States in 1895, 

 achieved his success only by making a very careful 

 study of and alterations in the telescope and its 

 accessories. While the changes he made were small, 

 they had, as he said, "greatly increased the practical 

 efficiency of the instrument, and therefore, small as 

 they are, they are important." Unfortunately, Keeler 

 died soon after he had commenced his photographic 

 study of the nebulae, but the handsome volume pub- 

 lished as a tribute to his memory (" Publications of 

 the Lick Observatory, vol. iii., 1908"), and containing 

 splendid reproductions from his negatives, will give 

 the reader some impression of the fineness of his 

 work. 



Beautiful as the photographs which up to the pre- 

 sent time have been secured are, there was inherent in 

 them some defects which it might have seemed im- 

 possible to eliminate. There is little doubt but that 

 all these photographs must now be consigned to the 

 second position, to be replaced by those that are the 

 work of Prof. G. W. Ritchey, of the Mount Wilson 

 Observatory. 



Prof. Ritchey is one of the band of valuable men 

 which Prof. George E. Hale was fortunate enough to 

 surround himself with in the establishment of the 

 Mount Wilson Solar Observatory. Prof. Ritchey was 

 previously one of the staff of the Yerkes Observatory, 

 and was in charge of the instrument shop at that 

 observatory, and this shop was regarded of very 

 great importance, since it alone rendered possible the 

 construction and frequent improvement of instruments 

 of new type or special design ; provision was also made 

 for optical work on a large scale. At the Mount 

 Wilson Observatory the instrument shop was naturally 

 of fundamental importance, and it was not long 

 before the figuring and mounting of a 5-foot re- 

 flector was undertaken. This instrument was first 

 tested visually in December of the year 1908, and the 

 first celestial photograph was secured in the same 

 month of that year. The instrument, mounting, 

 dome, building, and accessories were all carried out 

 from the plans of Prof. Ritchey, and it is with this 

 powerful instrument of research and close attention to 

 refinements that he has been able to make this pro- 

 gress in the photography of nebulae. 



In the efficient working of a reflecting telescope it 

 is of great importance to secure as far as possible 



NO. 2144, VOL. 85I 



equal temperature conditions for the telescope, dome, 

 and inside and outside air. Thus the telescope and 

 mirror must not be allowed to be heated up during 

 the daytime because change of temperature causes a 

 deformation of the reflecting surface of the mirror 

 and an alteration in the length of the telescope itself. 

 Again, bright sunshine on the dome causes the build- 

 ing to become heaicd, and this in turn affects the 

 telescope and mirror and produces temperature errors. 

 It is chiefly the elimination or practically the almost 

 complete elimination of such temperature changes that 

 has allowed Prof. Ritchey to secure his admirable 

 photographs, and a brief account of the way he has 

 achieved success will be of interest. In the first place, 

 tests were carried on in the optical shop to determine 

 how large a daily variation of temperature was per- 

 missible without seriously affecting the figure of so 

 large and thick (194 cm. at edge, ly^ cm. at centre) 

 a mirror. By allowing the air temperature about the 

 mirror to rise and fall uniformlv for twelve hours 



i 



Fig. I. — The 6o-inch reflector mounting in dome. 



respectively through 0° F. to 10° F., the most marked 

 effect on the mirror was a decided disturbance of the 

 figure on the outer zones of the surface for a distance 

 of 3'5 or 4 inches in from the edge, these zones be- 

 coming too high as the temperature rose, and reced- 

 ing and even becoming too low when the temperature 

 fell. The remaining zones were only slightly affected 

 and the change of focal length of the mirror was 

 small. It was finally decided that a daily variation 

 of the large mirror of 2° F. was the maximum varia- 

 tion that could be permitted without perceptible injury 

 to the sharpness of photographic star-images. When 

 it is mentioned that the daily variation of the tem- 

 perature in the unprotected dome in clear weather 

 in the latter half of June, 1909, at Mount Wilson 

 averaged 20° F., it will be gathered that the mirror 

 must have altered its figure very considerably. The 

 contraction of the steel skeleton tube was also very 

 noticeable, for the apparent change of focal length 

 found during the night frequently amounted to o"o4 

 inch. 



