ASTROPHOTOGRAPHY. The use of photographic methods in astronomical observa tions has very greatly increased both the ef fective power of astronomical telescopes and also the accuracy of astronomical measure ments.
By exposing a photographic plate to the light of a heavenly body, at the focus of a suitable telescope or camera, and giving the requisite motion to the telescope to compensate for the apparent motion of the heavenly body we are able to secure an autographic record of the form and details of that body, and also of its apparent position with reference to the sur rounding stars. In the case of celestial objects such as the nebula and the more distant stars, whose light is in most cases so faint that they cannot be properly studied or measured by visual observations even with the greatest tele scopes, long exposures of the photographic plate enable us to secure strong and distinct photo graphic records which can be studied and meas ured with ease and certainty. Furthermore, the more recent photographs show countless stars and countless details of nebula which are so faint that no trace of them can be detected visually with any telescope.
Photographic methods have been applied to solar and stellar spectroscopy with results fully as important and remarkable as in the case of the direct photography of stars and nebula.
These methods are described in separate ar ticles.
The present paper describes briefly the methods and instruments used in photographing the Milky Way or galaxy, star-fields, star clus ters, nebula, planets, comets and the moon. These methods will be described under the heads (1) astrophotography with refracting telescopes; (2) astrophotography with reflect ing telescopes.
The telescope is used as a great camera. The large lens, or in the case of a reflecting telescope the speculum, of the telescope serves as the lens of the camera; the tube of the telescope serves as the camera body or box; the photographic plate is placed at the focus of the lens or speculum. The telescope is moved slowly by a powerful machine called the driv ing-clock, so that it continually points accu rately at the apparently moving celestial body. In addition, a guiding or pointing telescope is usually attached rigidly to the tube of the pho tographic telescope, to enable the astronomer to watch throughout the exposure of the photo graphic plate, and to introduce minute cor rections of motion or position, when he sees that such corrections are necessary.
Astrophotography with Refracting Tel Among the earliest celestial pho tographs of great value secured with refracting telescopes were those made by Barnard, Rus sell, Wolf and others, using the type of refractor known as the portrait-lens or photo graphic doublet. This type of instrument pos sesses the advantage that it includes a large area of the sky in one photograph, with con siderable sharpness of detail all over; the best instruments of this class give a usable field of 10 or 12 degrees of arc in diameter, whereas the usual type of photographic refractor gives a usable field only one or two degrees in diameter. The astronomical photographic doublet is usually made of relatively small focal length, the ratio of focal length to aperture being or 5 to 1; this of course results in a relatively low magnifying power. In fact the great usefulness of this type of in strument is due primarily to the largt area of sky which it includes; and sharpness of detail is to some extent sacrificed in order to cover a very large field.
Using this type of instrument Barnard, Wolf, the astronomers of Harvard University Observatory, Franklin-Adams and others have been eminently successful. Barnard, using first the six-inch Willard doublet of the Lick Ob servatory, and later the 10-inch Bruce doublet of the Yerkes Observatory, has photographed the great star-clouds and the remarkable dark rifts of the Milky Way, the great areas of extended faint nebulosity and comets. Wolf at Heidelberg, using a pair of 16-inch photo graphic doublets, has made a study of all of the known large nebula and nebulous regions of the northern heavens. The Harvard University astronomers have covered the entire northern heavens again and again with similar instru ments of moderate power at Cambridge, and have also established an extremely powerful 24-inch telescope of this type at Arequipa, Peru, with which they have photographed the Milky Way, the Magellanic Clouds, the star clusters and the nebula of the southern celestial hemisphere. Franklin-Adams, 'using a 10-inch instrument, and working in England and in southern Africa, has included the entire north ern and southern celestial hemispheres in his series of photographs.