Astrophotography with Reflecting Tel For photographic purposes the re flecting telescope possesses several most im portant advantages over the refractor. First of these is its perfect achromatism; that is, in the reflector all colors or wave-lengths of light are brought to a focus in one and the same focal plane. A second advantage is that, in the case of the very large telescopes necessary in modern astrophotography, discs of glass of a quality suitable for reflectors can be secured much more easily and cheaply than those re quired for refractors; in practice this means that reflecting telescopes can be made of very much greater size and power than is possible in the case of refractors. A third advantage of the reflector is that a single instrument can be so designed and constructed that it can be quickly adapted to various focal lengths, and it is therefore readily available for the photog raphy of celestial objects requiring either low, intermediate or high magnifying powers. A fourth great practical advantage is that the reflector is relatively a short and compact form of telescope; this ensures comparative freedom from flexures which would affect the accuracy of the resulting photographs; and it greatly reduces the size and cost of the protecting dome and building.
About 1863, De La Rue in England and Draper in America first used reflecting tele scopes for celestial photography, chiefly of the moon; De La Rue using a 13-inch telescope with speculum-metal mirror and Draper a 16 inch with silvered glass mirror. About 1886 Common in England, using a 37-inch reflecting telescope with mirror by Calver, made the finest photographs of the larger and brighter nebula made up to that time. Common's success was so encouraging that he later constructed a 60 inch reflector, with which he secured many ex quisite photographs of nebula. A marked con tribution made to astrophotography at this time was the invention by Common of the double-slide plate-carrier in its simple form, for guiding during long exposures. In 1888 Roberts in England, using a 20-inch reflector with mir ror by Grubb, began a long series of photo graphs of nebula and star-clusters. Common's and Roberts' work with reflectors aroused pro found interest among astronomers, as did also the Henry brothers' work with refractors, for these were the first highly successful efforts in astrophotography with large instruments and with long exposures. Common's 37-inch re flector was bought by Crossley, and was pre sented by him to Lick Observatory in California, where about 1899 it was used by Keeler, with more refined methods than had hitherto been applied, in making a long series of exquisite photographs of faint nebula and star-clusters. As a result of these photographs Keeler was able to announce two discoveries of capital im portance: First, that the spiral nebula is the prevailing or most numerous type of nebulae; second, that hundreds of thousands of spiral nebula are within the reach, photographically, of such an instrument as the Crossley reflector.
Keeler's epoch-making work was interrupted by his death in 1900, but since has been continued at the Lick Observatory by Perrine, Curtis and others.
In 1901 Ritchey completed the 24-inch re flector of the Yerkes Observatory, and in 1901 and 1902 secured with it a series of photographs of the remarkable changing nebulosity around the star Nova Persei, a work which was simul taneously carried on by Perrine at Lick Ob servatory. This nebulosity was so faint that it was invisible in all visual telescopes, and could be photographed successfully only with the great short-focus reflectors. Later Parkhurst and others used the 24-inch reflector with great success in stellar photometry.
In 1908, under Ritchey's superintendence, the 60-inch reflector of the Mount Wilson Observa tory was completed and installed at the summit of Mount Wilson, California, nearly 6,000 feet above searlevel. This great instrument, de signed especially for celestial photography in many lines, is unique in many respects, two of which may be mentioned : first, nearly the en tire weight of the moving parts is floated in mercury, thus securing a very high degree of smoothness of motion in following the apparent motion of the heavenly bodies ; second, a series of interchangeable ends of the telescope tube allows any one of a number of small mirrors, plane and convex, to be used in combination with the great mirror; by this means the tele scope can be very quickly changed to any one of the various forms needed for the various kinds of work in celestial photography; the forms used are the Newtonian, the Cassegrainian and the Coude.
With this instrument used in a superb cli mate, and combining great size with great refinement of construction and the high photo graphic efficiency of the reflector, remarkable results have been secured in many lines of astrophotography. Adams and others have conducted epoch-making researches in stellar spectroscopy; Sears and Shapley have secured a long series of observations of the highest accuracy in stellar photometry; Van Maanen has made investigations of high accuracy in stellar parallax. Ritchey has made a series of photographs of spiral and other nebula and of star-clusters for the purpose of studying inter nal rotation and proper motion of these bodies. Fath and Pease have also made many fine di rect photographs of nebula, and have made very long-exposure photographs of the spectra of nebula and star-clusters.