395 



KDomledge & SeleDtJHe fleais 



A MONTHLY JOURNAL OF SCIENCE. 



Conducted by MAJOR B. BADEN-POWELL, F.R.A.S., and E. S. GREW, M.A. 



Vol. III. No. 17. 



[new series.] 



APRIL, 1906. 



SIXPENCE NET. 



CONTENTS^See page VII. 



Astronomical 



Photography. 



Hints to Amateurs Regarding 

 Apparatus and Methods of Working. 



By Alexander Smith. 



In dealing with astronomical photography from a 

 practical standpoint, it may be explained at the outset 

 that there is no single optical appliance suitable for all 

 classes of work. .Small nebula and dense stellar 

 clusters require a telescope of sufficient focal length to 

 give an image on a sufficiently large scale to exhibit 

 structure, or to resolve crowded clusters into their in- 

 dividual components. For example, a focal length of 

 eight feet is insufficient to resolve the central portion of 

 such objects as M. 13, the great cluster in Hercules, 

 while a similar instrument, when applied to some of the 

 small but well defined nebula; in the constellation \'irgo, 

 gives an impression on the plate no larger than that of 

 a single bright star. The result to be aimed at in deal- 

 ing with such bodies is to secure as large a photographic 

 image as possible, w'hich necessitates foca.1 length, and, 

 in order to bring the exposure within reasonable limits, 

 a correspondingly large aperture. 



The reflector with its single optical surface, absorb- 

 ing as it docs a minimum of light and giving at the 

 same time the requisite correction for chemical rays, 

 has so far proved the most popul.ir form of photo- 

 graphic telescope in the hands of both amateur and 

 professional, and the best all-round results have 

 probably been obtained with specula having a ratio of 

 aperture to focal length of f/4 or f/5. It has been 

 found that with the sensitive plates now in use a re- 

 flector having an angular aperture of f/4 will record the 

 faintest st u's visible in the largest telescopes with an 

 exposure of about i hour 30 minutes. If it is desired, 

 however, to obtain impressions of large diffused 

 nebulosities, such, for example, as that outside the 

 Pleiades group discovered by Professor Barnard in 

 December, 1893, on a pl.itCi which had received an 

 exposure of 10 hours, or of the large encircling nebula 

 in the constellation of Orion, :dso successfully photo- 

 graphed by the same astronomer, then the ordinary 

 telescope on account of its small field is altogether 

 useless, and recourse must be had to a lens of short 

 focus, which not only gives the requisite wider angle, 

 but at the same time greater rapidity. Dr. Max \Volf 

 has shown that his 2\ in. aplanatic doublet is five times 



more effective for the delineation of nebulae than the 

 13 in. Henry photographic refractor. On the other 

 hand, it was found to be 32 times its inferior in record- 

 ing impressions of stars (" Knowledge," December 2, 

 1895, p. 280). With such lenses — and those of the 

 Petzval and stigmatic types are more particularly re- 

 ferred to — much original work may still be done, and, 

 as they can be readily and effectively mounted on small 

 clock-driven equatorials — either reflector or refractor — 

 their use comes within the scope of amateurs, who may 

 be already provided with such equipments. With a 

 doublet lens of the portrait type, having an effective 

 aperture of from 2 to 6 inches, and a focal length corre- 

 sponding to an angular aperture of from f/3 to f/5, 

 exquisite photographs of extended nebulosities, wide 

 clusters, or of regions of the Milky Way can be readily 

 obtained. Such lenses are also highly suitable for 

 securing records of large comets, or of stray meteors, 

 which may happen to cross the region covered by the 

 plate at the time an exposure is taking place. 



In selecting a lens for any particular kind of work it 

 has to be kept in view that for photographing large 

 faint nebulous masses the duration of exposure 

 necessary to bring out a certain amount of detail is 

 very appreciably shorter with a small lens than with a 

 large one, notwithstanding that their angular apertures 

 may be precisely similar. This is due to the circum- 

 stance of the smaller lens presenting a greater amount 

 of contrast, and it is to this factor that the duration of 

 exposure largely depends. 



With points of light such as a star the duration of 

 exposure is determined by the intensity of the rays, and, 

 consequently, the contrast between the field and the 

 stellar image depends entirely on the angular aperture 

 of the lens, and the larger this is correspondingly 

 fainter stars come within its grasp for exposures of 

 similar duration. Of course, if the exposure is suffi- 

 ciently prolonged, a small lens would record as faint 

 stars as the largest telescope, provided the light rays 

 are of suflicient intensity to set up chemical action on 

 the plate. 



Put briefly, the smallest class of lens, say, from li to 

 3 inches of aperture, is the most eflertive optical ap- 

 pliance for the delineation of diffused nebulosities such 

 as those in Taurus or Orion already referred to, while 

 they are also well adapted for securing meteor trails. 

 The ckiss of phenomena coming within the reach of 

 lenses of from 3 to 6 inches aperture is verv much 

 wider, and embraces well-defined nebulous areas, open 

 clusters, regions of the Milky W'ay, stellar charts, 

 comets, &-C. If only a single lens is available it should 

 have as large a linear aperture as possible, and as large 

 an angular aperture as is consistent with good defini- 

 tion. For all solar, lunar, and planetary work, for 

 crowded stellar clusters, for small nebula? and all well- 

 defined objects of this class showing structure, re- 



