Alvan Clark (1804-1887) was one of the most famous telescope makers of the United States in the nineteenth century. Originally a portrait painter he built his first reflector in 1844 and two years later established a business with his sons Alvan Graham and George Bassett Clark. Together they made over 75 objectives which were over 6 inches in aperture, each of which was considered to be nearly optically perfect and among the best produced at the time.
Among their many achievements were the building of an 18.5 inch telescope for Evanston, two 26 inch lenses for observatories in Washington and Virginia, a 30 inch lens for Pulkowa in Russia, a 36 inch lens for the Lick Observatory and the largest lens in the world at that time, a 40 inch for the Yerkes Observatory in Chicago. Making telescope lenses was an exacting and time-consuming skill; the Washington lens for instance took over a year to grind and polish.
The time-consuming process used by Clark's when making their lenses is outlined in the following extract from Knight's 1874 American Mechanical Dictionary.
"The discs of Clark's lenses were made by Chance and Co. of Birmingham, England. The crucibles were made of clay, and built up gradually in rings of about 2 inches in height, the process requiring a whole year for its completion.
Optical glass of the best quality was then selected and crushed and the fragments separated according to their specific gravity by a hydraulic separator, in the manner employed for treating ores. Those of uniform quality and size are selected and melted by the most intense heat of a Siemens gas furnace; the mass is then cooled very slowly, and the central portion sawn out. This may be reheated until it is sufficiently fluid, and moulded to approximately the desired shape.
The discs are then tested, to ascertain if the glass is homogenous and free from flaws. This is affected by throwing the light from a lamp through a lens on one side of the disc, and placing the eye in the focus of the lens on the other side. Any imperfections thus appear greatly magnified, and if not removable by grinding, cause the rejection of the piece, at least for a lens of the size for which it was intended. The disc is ground upon concave plates of cast-iron of the proper curvature by pushing it back and forward at the same time giving a slow rotary movement. Emery, with water, is used as an abradant, finer sizes being successively used.
The polishing is effected by coating the tool with a thin layer of pitch, which is pressed into the proper shape; this is covered with rouge and water, and the disc manipulated as in the grinding process. The pieces forming the lens are, when finished, put together and set on edge, facing a luminous point placed at a distance equal to twelve to fifteen times the focal distance of the lens; the appearance of this point through the lens is examined with an eye-piece of high power, or by the eye placed in the focus; the optician thus judges what parts have an excess or deficiency of curvature. The polishing process is then repeated upon those portions as are too prominent."
Knight, E., H., (ed), 'Knights American Mechanical Dictionary', Vol III, J.B. Ford and Company, New York, 1874
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