Making Mirrors for Giant Telescopes

A walk through the evolution of large-mirror telescopes
14 August 2019
By Gwen Weerts
Hubert Martin OP19 plenary

In 1919, Edwin Hubble arrived at Mt. Wilson Observatory and started observing with the 100-inch (~2.5 m) Hooker Telescope. He obtained more distinct images of stars with the 100-inch telescope than with previous telescopes, and was able to identify a Cepheid variable star in Andromeda Nebula. This discovery expanded the known Universe by a factor of 10. His observations revolutionized cosmology.

This is what can happen when you build an instrument that can detect and measure things that could not be measured before—knowledge expands exponentially. In astronomy, these new and more sensitive instruments are usually telescopes, and they usually achieve greater sensitivity by using a bigger mirror. However, every increase in mirror size presents a new technical challenge.

Hubert "Buddy" Martin, project scientist for mirror polishing at the University of Arizona's Richard F. Caris Mirror Lab, walked through the evolution of large-mirror telescopes in his plenary talk on "Making Mirrors for Giant Telescopes" at SPIE Optics + Photonics 2019.

Martin discussed the evolution of lightweight material and honeycomb designs that enabled the design of behemoth telescopes like the under-construction 25-meter Giant Magellan Telescope, which uses seven 8.4 meter segments to make a near-parabolic primary mirror. This telescope design guarantees smooth wavefront over largest possible subapertures, and minimizes the number of elements that must be controlled. The mirror segments were spin-cast and polished at the UA Mirror Lab.

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