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How to make an Extremely Large Telescope

posted by Jason Kottke   Nov 09, 2017

The Giant Magellan Telescope, currently under construction at the University of Arizona’s Mirror Lab, will be one of the first of a new class of telescopes called Extremely Large Telescopes. The process involved in fashioning the telescope’s seven massive mirrors is fascinating. This is one of those articles littered with mind-boggling statements at every turn. Such as:

“We want the telescope to be limited by fundamental physics — the wavelength of light and the diameter of the mirror — not the irregularities on the mirror’s surface,” says optical scientist Buddy Martin, who oversees the lab’s grinding and polishing operations. By “irregularities,” he’s talking about defects bigger than 20 nanometers — about the size of a small virus. But when the mirror comes out of the mold, its imperfections can measure a millimeter or more.

Precision of 20 nanometers on something more than 27 feet in diameter and weighing 17 tons? That’s almost unbelievable. In this video, Dr. Wendy Freedman, former chair of the board of directors for the GMT project, puts it this way:

The surface of this mirror is so smooth that if we took this 27-foot mirror and then spread it out, from coast-to-coast in the United States, east to west coast, the height of the tallest mountain on that mirror would be about 1/2 an inch. That’s how smooth this mirror is.

You need that level of smoothness if you’re going to achieve better vision than the Hubble:

With a resolving power 10 times that of the Hubble Space Telescope, the GMT is designed to capture and focus photons emanating from galaxies and black holes at the fringes of the universe, study the formation of stars and the worlds that orbit them, and search for traces of life in the atmospheres of habitable-zone planets.

The telescope has a price tag of $1 billion and should be operational within the the next five years in Chile.