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E. F. Albin (Fernbank Science Center)
Between the years 1773 to 1818, Sir William Herschel constructed dozens of speculum telescope mirrors, with diameters ranging from 6 - 48 inches. Very little, if any, detailed information has ever been published on the specifics of his mirror figuring efforts. The reason for this certainly relates to his desire to closely guard mirror production trade secrets.
Upon Herschel's death, all telescope-making documents were passed on to his only son, Sir John Herschel. These materials are now in the possession of the British RAS and primarily consist of: a) a four volume series entitled "Experiments on the Construction of Specula," b) a 129 page treaty called "On the Construction of Specula," and c) a 179 page manuscript entitled "Results of Experiments on the Construction of Mirrors." It is suggested that publication was further delayed and then eventually abandoned due to silver-coated glass mirrors coming into favor.
A recent investigation by the author, of the unpublished manuscripts on the construction of specula, suggests that Herschel's mirror figuring techniques did not involve any guess work; in fact, his methods were highly refined -- never leaving to chance the evolution of a spherical surface into the required paraboloid. At the heart of Herschel's figuring techniques were a series of aperture diaphragms (similar to the Couder masks used by modern telescope makers) that were placed over the mirror, which allowed for the precise determination of its curvature at various predefined zones. With this information, Herschel was able to vary his figuring strokes with his polishing tool accordingly. In addition, all mirrors were subsequently "star tested," sometimes with aperture diaphragms in place, allowing for field examination of the mirror's "distinctness" or performance. Double stars and the planet Saturn were favorite targets used to analyze and then correct a mirror's figure.
The author(s) of this abstract have provided an email address for comments about the abstract: ed.albin@fernbank.edu
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Bulletin of the American Astronomical Society, 36 #2
© YEAR. The American Astronomical Soceity.