U of A to cast two mirrors in one for LSST

TUCSON, AZ—The University of Arizona’s (U of A) Steward Observatory Mirror Laboratory is casting a new kind of giant optic for a unique wide-field survey telescope called the Large Synoptic Survey Telescope (LSST).

TUCSON, AZ—The University of Arizona’s (U of A) Steward Observatory Mirror Laboratory is casting a new kind of giant optic for a unique wide-field survey telescope called the Large Synoptic Survey Telescope (LSST). The LSST will be built on Cerro Pachon, a nearly 2,700 m high mountain peak in northern Chile. Private and public partners, collaborating as the LSST Corporation, plan to begin the survey in 2014 or 2015.

The LSST will be the world’s largest, most powerful wide-angle survey telescope and will provide time-lapse digital imaging across the entire available night sky every three days, enabling astronomers anywhere simultaneous access to study supernovae, planet-approaching asteroids or comets, and other dynamic celestial chance events, and explore the nature of dark matter and dark energy. Normally, big telescopes see a patch of sky the size of a tiny piece of Earth’s moon. The LSST will see a section of sky roughly 40 times the size of the full moon and each image will be recorded at high resolution by a 3.2 billion-pixel camera arrayed in a 64 cm detector—the world’s largest.

The Mirror Lab will cast two mirrors as a single piece of glass (weighing 51,900 pounds)—an outer > 8.4 m diameter primary mirror and an inner 5 m diameter mirror in one mold. It is the first time a combined primary and tertiary mirror will be produced on such a large scale. The outer region of the primary mirror will collect celestial light and reflect it up to the separate 3.4 m diameter secondary mirror. The secondary mirror bounces light back down to the telescope’s tertiary mirror (monolithically attached to the primary mirror), which then sends it up again into a camera at the center of the secondary mirror. This complex down-up, down-up optical light path is needed to acquire the wide field-of-view.

“The conservative approach would have been to cast the first and third LSST mirrors separately,” said Mirror Lab director and Regents’ Professor of Astronomy Roger Angel. Ten years ago, he proposed the telescope design that has evolved into the LSST. “But it costs almost as much to cast a 5 m third mirror as it does to cast an 8 m primary,” Angel said. “If we put these in the same piece of glass, that saves how much glass you have to use all together, as well as the time it takes to cast two mirrors.”


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