The Dictionary Of Photography

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

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Astronomical Photography
the telescope as to keep the cord tense. A peg of wood in the hole b will serve to clamp the cord, although Lord Crawford used a clamping screw which drove the cord into a narrow groove, and so very accurately fixed its position. When the telescope is used as a camera a finder as shown in the sketch, is especially useful, although it is not difficult to so mount the original eye-piece of the telescope in the focussing screen frame of the camera that it will serve as a finder. To photograph a group of stars will generally involve an exposure of some minutes, and each star will be represented not as a point, but as a tracing of its apparent path. Such a stellar photograph has, as Dr. Stolze pointed out some years ago, the special advantage that a spot on the plate is not liable to be confounded with a stellar image. Photographs of the sun may be so instantaneous that the move-ment does not count.
Exposure when the Moon is Photographed. Writing in The Amateur Photographer for July 9th, 1897, Dr. Woolsey Blacklock says : - " I would suggest that the maximum exposure with a fixed telescope is half a second, if the image is intended to be enlarged to i\ inches, and it ought to bear this. The angular diameter of the moon may be taken roughly as half a degree, and it moves the same distance in two minutes, or 240 half-seconds, therefore if the final image is intended to beinch diameter, orof an inch, it will practically move
inches in half a second, and a longer exposure will cause blurring. This exposure is ample for the full moon in winter, but scarcely enough for the craters near the terminator at the quarters."
Astronomical Work with an Ordinary Camera. Mr. Barker North treats of this subject in The Amateur Photographer for December 26th, 1901 (p. 514), and he points out that, for purposes of measurement, star trails are often more convenient than point-like images. He gives a photograph showing the trails of the circumpolar stars. In a subsequent issue (Jan. 9th, 1902, p. 22), Mr. E. W. Maunder emphasises the value of the star-trail method with an ordinary camera, and he points out that, by interrupting the exposure for about 20 seconds, the time of any events {e.g. a meteor) may be registered. Further, it is pointed out how the trail method with gaps may prove valuable as a means of automatic record.