STARS, PHOTOGRAPHING The various methods of photographing stars are so numerous that the most important of them will be best treated under special headings, as noted below. In general the science of astrophotography may be divided into two main classes, depending on whether the photographic camera is stationary, or is moved either by hand or clockwork so that the apparent motion of the stars across the field is neutralised.
Astrophotography with Stationary Camera.— There are numerous subjects with which valuable results may be obtained with no extra apparatus beyond an ordinary camera. Suppose the photographer wishes to prepare for himself an accurate chart of a certain constellation of stars. All that is necessary is to direct the camera, either by means of the tripod sliding legs or by some form of tilting table, to the middle of the constellation. The plates em ployed should be of the most rapid variety obtainable, and preferably panchromatic or uni formly colour-sensitive. When all is quite rigid, give an exposure of, say, ten or fifteen minutes with the largest stop available. On developing the plate a series of arcs of circles having different densities will be seen. These are the " trails " of the stars which were projected on the plate, the movement being due to the earth's rotation past the star during the exposure. Obviously, then, the length of the lines will depend on the length of exposure. If, now, the beginning or ending of every star trail is marked with a small dot of Indian ink, of the same diameter as its corresponding line trail, the resulting series of dots will show as a star map. If the accompanying lines are found to be confusing, it is quite simple to trace the dots on tracing paper or cloth, when a star map will be obtained which may be placed in front of a small box containing a lamp. This contrivance will be found of great help to anyone wishing to teach the forms of the constellations, as the illuminated stars may be easily seen by a number of people at the same time.
Numerous other methods of utilising photo graphs obtained by fixed cameras will doubtless occur to the worker. For example, at the times of the year when it is known that a shower of meteors is expected, the camera may be directed to the proper position and left open for some time in the hope of a meteor flashing across the field of the lens. Many interesting pictures of these remarkable celestial visitors have been thus recorded. (See also " Meteor Photo graphy.") Astrophotography with Moving Cameras.—When it is desired to obtain accurate charts of the sky, the camera must be so mounted that by means of suitable clockwork the stars may be kept exactly on the same portions of the photo graphic plate throughout the exposure. The motion of the stars from east to west across the sky is at such a rate that a complete circuit of the heavens is made once in 24 hours. If the direction of motion be carefully examined, it will be found that all the stars travel in circles parallel to the equator, so that to follow them we must employ what is called an equatorial mounting ; this consists of an arrangement whereby the axis of rotation can be tilted with respect to the horizon, until at any given place it is exactly parallel to the earth's axis. When this is accurately adjusted and the clockwork rated properly, it will be possible to obtain star photographs with any desired length of exposure.
With the apparatus above described it becomes possible to employ very powerful cameras, both as to aperture and focal length, so as to record stars so faint that they have never been seen by the unaided eye. The telescopic star cameras used in this way are of two kinds, depending on whether a refracting or reflecting system is used to give the image. The largest refracting lens telescope existing is that made for the Paris Exhibition of igoo, with a lens of 5o in. aperture and 18o ft. focal length. This is not in use, however, owing to the great expense of working.