Image of Mirror here
For many decades it was thought impossible to build a telescope as large as Gemini because the main (primary) mirror would have to be excessively thick and heavy to maintain its precise shape. Now, thanks to new technology, Gemini uses a relatively thin primary mirror that is able to hold its precise shape with a little help.
Mounted behind the mirror are 120 "actuators" that constantly nudge the mirror back into perfect form. These adjustments are typically only about 1/10,000 the thickness of a human hair and are enough to keep starlight precisely focused so astronomers can study the universe.
Using this technology Gemini should lead the way for a whole new generation of telescopes that will be larger than anything ever imagined by astronomers even a decade ago.
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