April io, 1919] 



NATURE 



105 



THE DOMINION OF CANADA'S 72-IN. 

 TELESCOPE, 



A LTHOUGH the reflecting^ telescope of the 

 ■^^ Dominion Astrophysical Observatory, Vic- 

 toria, B.C., is exceeded in size by the 

 loo-in. Mt. Wilson reflector, now nearly 

 completed, it has had the distinction of being 

 for some months the largest in operation in 

 the world. The Government of Canada is to 

 be congratulated on carrying through to comple- 

 tion during the war this great undertaking, 



Fii.. 1. — The observaiory 1 iiildiiig fruiii ihc south. 



which gives every ' promise, so far as quality 

 and efficiency of the equipment are concerned, 

 of being a very large factor in astronomical 

 research. 



A preliminary description of the mounting of 

 this telescope was given in Nature of Feb- 

 ruary 15, 191 7, but its final completion last May, 

 and its continuous use since then in. regular 

 observ'ational work, merit a short statement of 

 the quality of the optical parts and of the work 

 being done and proposed to be done with this 

 splendid instrument. 



NO. 2580, VOL. 103] 



The mounting was completely erected in its 

 dome and building on Observatory Hill (Fig. i), 

 about eight miles north of the city of Victoria, in 

 October, 1916, but the principal mirror and other 

 optical parts of the telescope were not finished 

 until April, 1918. The delay was due partly to 

 the impossibility of obtaining a large disc of glass 

 for an auxiliary flat to be used in testing the figure 

 of the paraboloid, and partly to the increased diffi- 

 culty in figuring caused by the presence of the 

 central hole in the main mirror. However, the 

 figuring was finally completed early in April, 

 1918, and on testing the mirror 

 at the centre of curvature by 

 visual measurements of the 

 radius of curvature of several 

 zones of the surface, and also by 

 means of the Hartmann method 

 of extra-focal photographic ex- 

 posures, the whole surface was 

 found to be remarkably close to 

 the required theoretical form. 

 The deviations of any part no- 

 where correspond with a greater 

 longitudinal aberration at the 

 principal focus than 025 mm. 

 (001 in.), and this for a median 

 zone. This is equivalent to a 

 lateral aberration of less than 

 one-tenth of this amount, or to a 

 circle of confusion less than one- 

 ihousandth of an inch in dia- 

 meter, which, bearing in mind the 

 size of the mirror, is a remark- 

 able perfection of figure. 



The mirror with other optical 

 parts, which arrived in Victoria 

 on Apriji 29, was installed and 

 collimated an<l the first star spec- 

 trum obtained on May 6. Con- 

 sidering the size and hitherto un- 

 tried features of the telescope, 

 this speaks well for the care used 

 in the design and construction of 

 Ix)th optical parts and mounting. 

 The instrument has been used 

 continuously since, mostly in ob- 

 taining stellar spectra, and has 

 given the utmost satisfaction. 



The tests of the figure of 

 the mirror were obtained in 

 the optical shop under con- 

 stant temperature conditions, and 

 it was of interest to determine its be- 

 haviour under average observing conditions 

 in its dome. Although one of the reasons 

 for the choice of Victoria as a site for the tele- 

 scope was the low diurnal range of temperature, 

 the total range in twenty-four hours rarely exceed- 

 ing 5° C, yet it was soon seen that even a 

 smaller change than this introduced considerable 

 aberration in the figure of the mirror. Hartmann 

 tests made after a daytime rise of about 5° C. 

 showed a longitudinal aberration, under correc- 

 tion, of nearly 3 mm., as compared with 0*25 mm. 



