Mirrors for large telescopes
There are some very large optical telescopes in the world. The newer ones employ complex and cunning tricks to maintain their precision as they track across the sky—more on that in the postscript. But if you want a simple telescope,youmakethereflectorasasinglerigidmirror.Thelargestsuchtelescopeis sited on Mount Semivodrike, near Zelenchukskaya in the Caucasus Mountains of Russia. The mirror is 6m (236in.) in diameter. To be sufficiently rigid, the mirror, which is made of glass, is about 1m thick and weighs 70 tonnes. The total cost of a large (236 in.) telescope is, like the telescope itself, astronomical—aboutUS$280m.Themirroritselfaccountsforonlyabout5percent ofthiscost;therestisthatofthemechanismthatholds,positions,andmovesitas it tracks across the sky. This mechanism must be stiff enough to position the mirrorrelativetothecollectingsystemwithaprecisionaboutequaltothatofthe wavelength of light. It might seem, at first sight, that doubling the mass m of the mirror would require that the sections of the support-structure be doubled too, soastokeepthestresses(andhencethestrainsanddisplacements)thesame;but the heavier structurethen deflectsunderits own weight. In practice,the sections have to increase as m2, and so does the cost. Before the turn of the century, mirrors were made of speculum metal (density: about 8Mg/m3). Since then, they have been made of glass (density: 2.3Mg/m3), silvered on the front surface, so none of the optical properties of the glass are used. Glass is chosen for its mechanical properties only; the 70 tonnes of glass is just a very elaborate support for 100nm (about 30g) of silver. Could one, by taking a radically new look at materials for mirrors, suggest possible routes to the construction of lighter, cheaper telescopes?