Speculum metal
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Speculum metal is a mixture of around two-thirds copper and one-third tin, making a white brittle alloy that can be polished to make a highly reflective surface. It was used historically to make diffe
The speculum metal mirror from William Herschel's 1.2-meter (49.5-inch) diameter "40-foot telescope" at the Science Museum in London
Modern replica of Herschel's 6.2"-aperture Newtonian telescope which had a speculum metal main mirror
Leviathan telescope. Telescopes with speculum metal mirrors were a large breakthrough in aperture, but their drawbacks fueled competition from refractors
The metal mirror of the Leviathan, the largest telescope mirror until the 100-inch Hooker telescope of 1917 (a metal-on-glass mirror)
First-surface mirror
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A first-surface mirror or front-surface mirror is a mirror with the reflective surface being above a backing, as opposed to the conventional, second-surface mirror with the reflective surface behind a
Example of a second-surface mirror (left) and a first-surface mirror (right). In both pictures, the pen is touching the surface of the mirror. "Ghosting" (a faint reflection from the first layer) is visible in the left image (more obvious when this file is displayed at full-size).
Technicians assemble 6 of the 18 first-surface mirrors used in the James Webb Space Telescope.