166 



NATURE 



[April 29, 1920 



Some Tests of the 100- in. Hooker Telescope. 



By Dk. George E. Hale, For.Mem.R.S., 



''T^HE construction of a telescope of very large 

 -»- aperture is necessarily an experiment the 

 final success of which can be determined only by 

 the results of astronomical observations. After 

 the mechanical and optical difficulties have been 

 overcome, there remain those disturbances of the 

 atmosphere which are of little importance with 

 small apertures and low magnifying powers, but 

 become more and more serious as the diameter 

 of the objective and the scale of the image are 

 multiplied. Thus in undertaking the construction 

 of a reflecting telescope of loo-in. aperture, while 

 we had the advantage of experience with the 6o-in. 

 Mount Wilson reflector, we frankly recognised 

 that the outcome must remain in question until 

 the completion of tests made under customary 

 atmospheric conditions. 



As contrasted with the 60-in. reflector, the full 

 measure of advantage attainable under perfect 

 atmospheric conditions would be as follows : 



60-in. reflector 

 loo-in. ,, 



Light 



collecting 



power 



Scale of 

 focal 

 image 



I 

 17 



Resolving 

 power 



I 



This means that either in direct photography or 

 in spectrographic work with a given dispersion the 

 larger instrument should bring within range stars 

 about one magnitude beyond the reach of the 

 60-in. The advantage thus gained would be im- 

 portant, as two illustrations, out of many that 

 might be given, will suffice to show. Only two or 

 three of the brightest stars can be studied with 

 the most powerful spectrograph of the 60-in. tele- 

 scope, which is not much inferior in dispersion to 

 the instrument used by Rowland in his work on 

 the solar spectrum. The same high dispersion, 

 if employed with the loo-in. reflector, could be 

 applied to several times as many stars, represent- 

 ing most of the principal stages of stellar evolu- 

 tion. At the other end of the magnitude scale, 

 the loo-in. telescope should be able to record 

 photographically many millions of stars too faint 

 to be reached by the 60-in. 



In the second place, the increased scale of the 

 focal image, whether in the principal focus of 

 the large mirror or at the Cassegrain focus, where 

 the loo-in. telescope has an equivalent focal 

 length of 134 ft., should separate more widely the 

 crowded stars at the centre of globular star 

 clusters, thus permitting them to be studied with- 

 out confusion with one another ; it should increase 

 the precision of measurement in such difficult 

 work, for example, as that involved in detecting 

 the very small changes in the configuration of 

 spiral nebulae caused by their internal motions ; 

 and, to give no other illustrations, it should permit 

 minute details, not previously photographed, to 

 be recorded on negatives of such objects as the 

 moon. 



Finally, the increased theoretical resolving 

 power, if realisable in practice, should permit the 

 NO. 2635, VOL. 105] 



Director of the Mount Wilson Observatory. 



visual measurement of the components of very 

 close binary stars, which cannot be separated by 

 the smaller instrument, not merely because of the 

 overlapping of their images during periods of 

 poor seeing, but also because of their inherent 

 irresolvability due to the wave-length of light 

 itself. 



In spite of the fact that most of the tests have 

 been applied under the comparatively poor at- 

 mospheric conditions of the winter season, the 

 early results are very satisfactory. It has become 

 possible to include in our regular spectrographic 

 programme stars one magnitude below those 

 studied with the 60-in,, so that the radial veloci- 

 ties and spectroscopic parallaxes of stars down 

 to the tenth magnitude are being measured in 

 large numbers by Dr. Adams and his associates. 

 This involves a notable extension of the range of 

 our investigations on the structure and motions of 

 the stellar universe. Similarly, an important ad- 

 vance in our researches on stellar evolution has 

 also been rendered possible, bringing to light new 

 and curious types of stellar spectra and interest- 

 ing phenomena in the spectra of variable stars at 

 minimum brightness. The spectra of the long- 

 period red variables of the Md class, most of which 

 were too faint for satisfactory observation with 

 the 60-in., are now being systematically photo- 

 graphed by Dr. Merrill with the Hooker telescope. 

 One of these stars has been found to show the 

 chief nebular lines in its spectrum, a matter of 

 peculiar interest because of the fact that these lines 

 have previously been observed only in nebulae and 

 in temporary stars. With low dispersion, the 

 spectra of stars of the fourteenth magnitude have, 

 been photographed by Dr. Shapley with moderate 

 exposures at the centre of globular clusters. 



The preliminary results of the photography of 

 nebulas have also been very satisfactory, both at 

 the principal (Newtonian) focus of the lOO-in. 

 mirror and at the 134-ft. Cassegrain focus. The 

 photographs indicate an important advance over 

 the 60-in. telescope, and leave no doubt that the 

 desired increase in the precision of measurement, 

 rendered possible by the larger scale of the images, 

 will aid materially in the study. of the internal and 

 proper motions of spiral nebulae. A striking 

 feature of these negatives, as compared with those 

 taken with the 60-in., is the increased contrast of 

 the minute nuclei in spiral nebulae, which are 

 brought into greater prominence by the larger 

 aperture. This will render available for measure- 

 ment a large number of sharply defined symmetri- 

 cal points. 



The character of the images may be judged from 

 the accompanying reproduction of a photograph of 

 the moon (Fig. i), taken by Mr. Francis G. Pease 

 at the 134-ft. Cassegrain focus on September 15, 

 1919. This negative, like others obtained by Mr. 

 Pease with the Hookef telescope, shows smaller 

 details of structure than we have previously been 

 able to photograph. 



