STELLAR PHOTOGRAPHY FOR AMATEURS. 



By W. H. STEAVENSON, F.R.A.S. 



Considering the large and important part now 

 played in astronomy by the photographic plate, 

 it is strange that comparatively few amateurs are 

 actively interested in this branch of the science. 

 No doubt most users of telescopes have tried their 

 hand at photography at some time or another, but 

 such attempts are too often limited to the taking of 

 a few snapshots of the sun or moon in the principal 

 focus of the instrument, which is then applied once 

 more to some line of visual work with results that 

 are generally of questionable value. Without 

 wishing for one moment to discourage useful 

 visual work, it is the aim of the writer to show 

 the great possibilities of stellar photography from 

 the point of view of the amateur and to suggest a 

 few branches of astronomical work to which he may 

 profitably apply it. 



A few words on the subject of equipment may not 

 be out of place. A common mistake on the part of 

 beginners is to attempt work on too large a scale, 

 using the biggest apertures and longest foci at their 

 command. After what .has been done by Roberts, 

 Keeler, and Ritchey, however, it is doubtful whether 

 many amateurs could make any useful advance in the 

 photography of individual clusters and nebulae. It 

 is in small-scale and wide-field work that the average 

 amateur will find it most profitable to engage. The 

 exact size and make of the lenses used must, of 

 course, be largely a matter of individual choice ; but 

 whatever choice is made there are at least two qualities 

 which a lens to be used for stellar work should 

 possess, namely, great rapidity and great covering 

 power. Slow lenses may, of course, be used, but 

 the necessary exposures will be so long that it will be 

 hardly worth while to use a lens whose aperture is 

 less than /5 or /6. Unless expense is no object it is 

 perhaps best not to try to combine extreme rapidity 

 and good covering power in one and the same lens. 

 It is much better, if possible, to have two lenses — a 

 portrait lens for rapid work on small areas and an 

 anastigmat for large fields. The portrait lens should 

 have a working aperture of /3 or /4, and should be 

 of at least twelve inches focus. Such lenses are 

 often to be picked up quite cheaply in second-hand 

 shops. Their covering power is rather poor, and if 

 the focus is not above twelve inches or so there is 

 no point in using anything larger than a quarter- 

 plate with them. As a rule their limit of fair 

 definition does not extend beyond a circle eight or 

 ten degrees in diameter. The anastigmat should be 

 of about eight inches focus, should work at /4 • 5 or 

 /4-8, and should cover a half-plate sharply at its 

 full aperture. Under such conditions the lens will 

 cover an"area in the sky of thirty by forty degrees, 



or considerably more than four times the area 

 enclosed by the four chief stars of Orion. The 

 portrait-lens and the anastigmat may be used simul- 

 taneously, thus acting as mutual checks on each 

 other's work. If they are used separately, the 

 former will obviously be employed for detailed work 

 on small areas, while the latter will be used when it 

 is necessary for any purposes to cover a large region 

 of the sky at once. Of course, for all photographic 

 work necessitating long exposures an equatorial 

 stand is absolutely essential ; but it may be a very 

 simple one, and need not even be provided with a 

 clock. The latter adjunct is an obvious advantage 

 as a saver of labour, but, given the requisite patience 

 on the part of the operator, excellent results may 

 easily be obtained with hand driving alone. Unless 

 the telescope is a large one it is best to attach the 

 cameras as counterpoises to the instrument, as this 

 saves unnecessary weight, which is apt to militate 

 against smooth and easy driving. For guiding, the 

 out-of-focus image of a star, bisected by cross-wires 

 in the eyepiece of the telescope, should be used. 

 In order to correct for refraction, a slow motion in 

 declination is essential. Care should be taken to 

 follow accurately to within a few seconds of arc, as 

 even small errors are much more noticeable on 

 photographs than might be expected. 



It now only remains to indicate in a general way 

 some of the uses to which such lenses as those 

 mentioned above may be put. First there is the 

 search for variables and novae. In this branch of 

 work it is best not to try to cover too much ground 

 at once, but to work systematically on one small 

 region at a time, preferably with the portrait-lens. 

 A small part of the sky, in or near the Milky Way, 

 should be selected and photographed repeatedly 

 with the same instrument at intervals of a few 

 days, the plates being carefully compared by super- 

 position or some other method. For the detection 

 of short-period variables it is best to make several 

 exposures of equal length and at equal intervals on 

 the same night, moving the camera slightly for each 

 exposure. The result will be a number of images of 

 each star, which will be of equal intensity in every 

 case, unless there is in the region a variable under- 

 going rapid change at the time, in which case it can 

 at once be detected by the inequality of its various 

 images. This method has been used with good 

 results by Professor E. C. Pickering at Harvard. 



Another use for the photographic lens is in the 

 search for comets. For this the anastigmat, on 

 account of the large field it covers, is especially 

 suitable. A region of the sky not too far from the 



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