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or Astronomical Photography

telescope, photographic, visual, photograph, plate, image and light

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ASTRONOMICAL PHOTOGRAPHY, or ASTRO-PHOTOGRAPHY.—In the very earliest days of photography attempts were made to photograph celestial bodies. In 1838 Daguerre attempted to photograph the moon, but was unsuccessful. In 1840 some experiments by Dr. Draper, of New York, resulted in the production of lunar photographs 1 inch in diameter. In 1850 Professor Bond, of Cambridge, Mass., secured with the Harvard 15 inch refractor a daguerreotype of the moon, which was exhibited at the great London Exhibition in 1851, and caused no little sensation, as it was the first time that a distinct image of the moon's surface had been obtained. With the same instrument he secured a series of photographs of the double star Mizar. In 1853 Mr. Warren de la Rue obtained very successful results in lunar pictures, and in 1857 he constructed an observatory at Cranford, expressly for celestial photographic purposes.

With regard to the telescope used for photographic purposes, Prof. Barnard says :* "It is perhaps not generally known that a visual and a photographic telescope are quite distinctly different in the manner in which they deal with the light of an object. To be brief, it is only necessary to say tnat tne visual telescope Is correctea tor tnat xina or ugnt wnicn gives tne brightest image, but this comes from a portion of the spectrum that has no effect on the ordinary sensitive plate. Hence a visual telescope cannot be used successfully for photographing since the image, so distinct to the eye, cannot he photographed. In the photographic telescope, however, the image cannot be seen because it is formed from that portion of light which, though it has the strongest action on the photographic plate, does not have any effect on the eye. Or, more briefly still, the visual telescope is useless for photography, and the photographic telescope cannot be used for visual work. Hence, also, the reason that for photography two telescopes are used—one to photograph with and the other to see to guide by. But the Lick telescope really consists of the two kinds of telescope in one. That is, by the addition of a photographic correct ing lens placed over the visual objective it can at once be converted into a photographic telescope.

It is then only 33 inches in diameter—the correcting lens being smaller than the visual. The "guiding" for this instrument consists not in delicate adjustments to move the entire telescope accurately with the stars, but in moving the plate itself to correct irregularities of refraction, etc., during the exposure—the clock, in the meantime, carrying the telescope as best it can." Although astronomical photography was at first regarded more in the light of curiosity than of utility, the advent of the collodion process, and afterward dry plates, changed all this, and there is no science that has benefited by the aid of photography to a greater extent than astronomy.

With regard to solar photography, the first attempt was made at Paris in 1845, by MM. Foucault and Fizeau, acting on the suggestion of M. Arago. Later, in 1851, attempts were made to photograph the total eclipse. In 1857 Warren de la Rue was commissioned by the Royal Society to construct an instrument specially adapted for photographing the sun at Kew Observatory. The resulting photo-heliograph may be described as a small telescope of 3i inches aperture and 5o inches focus, with a plate-holder attached to thee ye-end, and guarded in front by a spring slide, the rapid move ment of which across the field secured for the sensi tive plate the virtually instantaneous exposure neces sary, owing to the enormous light power of the sun. By means of this instrument the first solar pictures of value were taken, and a photographic record of the solar condition, recommended by Sir John Her schel, was begun, and continued to be carried on at Kew for fourteen years, 1858-72.

The first really good photograph of the camera, however, was made in 1869, at the total eclipse of that year, by Prof. Stephen Alexander.t The work of photographing the sun is now carried on in every part of the world, and there are few days on which the self-betrayal of the camera can be evaded by our luminary.

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