The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Astronomical Photography
Astronomical Photography. This is to a great extent beyond the reach of the ordinary worker, as the size of the images given by ordinary photographic lenses is excessively small, being, in the case of the sun or moon,of an inch in diameter for every ten inches of focus, but with Dallmeyer's telephotographic lenses considerable augmentation of the size is obtained ; on the other hand, when it is desired to obtain negatives of any portion of the sky, portrait lenses of about 6 inches aperture and 30 inches focus may be used for half or whole plates, which will give fairly successful results. A useful article on this subject will be found in Astronomy and Astrophysics, October 1892, p. 641. Pos-sessors of aor 3-inch refracting telescope may, however, wish to utilise the same for lunar or stellar work, and in such a case it is necessary to remove the eye-piece, and attach a very light camera in its place, the operation of focussing and exposing being the same as usual, using very rapid plates, and giving exposures of about; longer than this will cause blur-
ring of the image, due to the combined movements of the earth and moon. As astronomical telescopes are corrected for the visual and not the chemical rays, it is necessary to find out experimentally the difference ; and this is best done by focussing with the eye as sharply as possible, and then racking the plane of the sensitive surface further out by sixteenths of an inch, making an exposure after each movement till the sharpest image is obtained on the negative after development. The distance thus found may be once for all marked on the draw-tube of the telescope. A simple and remarkably ingenious device, described fry Lord Crawford about twenty years ago, serves to transform the pillar and claw stand into a form of equatorial, very serviceable for such work as the ordinary amateur is likely to undertake ; any object in the great revolving sphere of the heavens being now easily followed by one movement of the telescope or camera, instead of the double movement otherwise necessary. The sketch shows a form of Lord Crawford's device, easily arranged by any one possessing a small telescope and stand of the usual pattern. In the first place the telescope stand must be mounted on a firm slab or bench, as shown in our sketch, the bench being adjusted as accurately as possible north and south. When we say the telescope stand is to be mounted on the slab, we mean that provision should be made for readily mounting it, as the