Clark, Alvan

views updated May 29 2018

Clark, Alvan

(b. Ashfield, Massachusetts, 8 March 1804; d. Cambridgeport, Massachusetts, 19 August 1887); CLARK ALVAN GRAHAM (b. Fall River Massachusetts, 10 July 1832; d. Cambridgeport, Massachusetts, 9 June 1897); CLARK GEORGE BASSETT (b. Lowell, Massachusetts, 14 February 1827; d. Cambridgeport, Massachusetts, 20 December 1891),

astronomical instrumentation

Three instument markes—Alvan Clark and his sons, George Bassett and Alvan Graham—figured importantly in the great expansion of astronomical facilities that occurred during the second half of the nineteenth century. Almost every American observatory built during this period, and some observatories abroad, housed an equatorial refracting telescope and often auxiliary apparatus as well, made by the Clarks. Five times the Clarks made the objectives for the largest refracting telescope in the world; and the fifth of their efforts, the forty-inch lens at the Yerkes Observatory, has never been surpassed. Their optical work, which was recognized as unexcelled, was the first significant American contribution to astronomical instrument making. American telescopes had been made before, but none compared with those of European manufacture; by the end of the nineteenth century, however, partly because of the example set by Alvan Clark & Sons, several other Americans were making fine astronomical instruments that did indeed compare.

In their factory at Cambridgeport, Massachsetts, Alvan and Alvan Graham specialized in optical work, while George supervised the mechanical constructions. Most of the telescope produced were visual achromats, with objectives patterned after Fraunhofer’s lenses; the Clarks did not join in the contemporary mathematical search for more perfect lens configurations. The preliminary lens grinding was done by workmen using simple horizontal turntables. Then Alvan or Alvan Graham would locate the errors, usually by examining the image of an artificial star at the focus of the lens, and remove them by “local correction”—by manually retouching the offending areas.

Before the development of the Hartmann test the standard way of describing the perfection of a lens was in term of its actual defining power. The Clarks tested their lenses by searching for new and difficult double stars. In 1862, while testing the 18½ inch objective later installed in the Dearborn Observatory in Chicago, Alvan Graham discovered the predicated but hitherto unseen companion of Sirius; for this find he won the Lalande Prize of the Paris Academy of Sciences.