Of the three giant (or 'Extremely Large') new telescopes being built, the Thirty-Meter (TMT), the the Giant Magellan (GMT) and the Extremely Large Telescope (ELT), only the Magellan, from what I understand, will not feature a Cassegrain-type telescope, or some variant thereof.

Instead, the GMT will have a 'Gregorian'-style telescope, which is supposedly a predecessor of the Cassegrain....

But... What, exactly, is the difference?


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