E. F. M. Albin (Fernbank Science Center)
Although many of Sir William Herschel's telescope designs did not require a secondary mirror, he did construct Newtonian style optical systems that used "plain mirrors."
A look at Herschel's own writings on the matter gives some interesting insights into the techniques of this skilled 18th - 19th century telescope maker. The author provides glimpses acquired from unpublished Herschel documents that are now in the possession of the British RAS -- these include a four volume series entitled "Experiments on the Construction of Specula," a 129 page treaty called "On the Construction of Specula," and a 179 page manuscript entitled "Results of Experiments on the Construction of Mirrors."
Herschel constructed secondary mirrors for his instruments by grinding and polishing his small mirrors in a sequence that involved two tools. He tested the optical quality of a small plain mirror by utilizing two slips of white card or pasteboard. One slip was made exactly twice the length of the second. The shorter card was placed against the mirror while the longer placed at some distance away so that its reflected image exactly matched the other. The larger card was carefully placed near the eye and oriented so that it was parallel to the shorter card strip. In order to test various sections of the secondary mirror, cards of different length were used. The mirror was judged flat if the reflected image was exactly identical to the card resting on the mirror. If the mirror were concave, the reflected image would be larger (i.e., magnified) than that of the smaller slip. However, should the reflected image be lesser in size, then the mirror was convex. Figuring was done with the mirror resting above the polishing tool. A convex mirror could be made plain by lengthening the stroke while a concave mirror could be made plain by shortening the stroke.
The author(s) of this abstract have provided an email address for comments about the abstract: ed.albin@fernbank.edu
Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.