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Giant optics gives telescope a wider view

19 Mar 2008

The world's most powerful wide-angle survey telescope uses a huge two-in-one mirror to become the widest, fastest and deepest eye of the new digital age.

The casting of a giant two-in-one mirror will enable Arizona's Large Synoptic Survey Telescope (LSST) to capture the entire available night sky once every three days. The ground-based telescope will provide time-lapse digital images that will allow astronomers to track moving objects such as planet-approaching asteroids and comets.

Each image will be recorded at high resolution by a 3.2 billion pixel camera arrayed in a 64 cm detector. The unique optics will allow the LSST to see a section of sky roughly 40 times the size of the full moon, while conventional large telescopes are only able to image sections of sky just of a fraction of the size of the moon.

Three mirrors for the LSST will be cast at the University of Arizona's Steward Observatory Mirror Laboratory: an 8.4 m diameter primary, a 3.4 m diameter secondary and a 5 m diameter tertiary. Key to the telescope design is the casting of the primary and tertiary mirrors using a single piece of glass in the same mould. This approach reduces cost and is the first time that a combined primary and tertiary mirror will be produced on such a large scale.

"It costs almost as much to cast a 5 m tertiary mirror as it does to cast an 8 m primary," said Roger Angel, the Mirror Lab's director, "If we put the mirrors in the same piece of glass, we save on the amount that we have to use all together, as well as the time it takes to cast two mirrors."

Another advantage of producing the two mirrors using one piece of glass is that they can be precisely aligned in the laboratory. "We save money both in the manufacture of the mirrors and also over the lifetime of the telescope because of the simplicity of permanently holding the mirrors in proper alignment," said Angel.

In the telescope setup, the outer region of the primary mirror will collect celestial light and reflect it up to the separate secondary mirror. The secondary mirror bounces light back down to the telescope's tertiary mirror, which then sends it up again into a camera at the centre of the secondary mirror. This complex optical light path is needed to acquire the wide field of view.

Production of the combined primary and tertiary mirrors began this month and involved loading 51,900 pounds of borosilicate glass into a giant rotating furnace. The furnace, which has a diameter of 12 m, spins at almost seven rotations per minute for over three days, during which time the spinning molten glass will take on the curve of the primary mirror by centrifugal force. Because the tertiary mirror has a deeper curvature, extra glass in the tertiary mould will be ground away later.

The LSST is scheduled to begin its survey of the sky in 2014 or 2015.

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