- Primary mirror
A primary mirror (or primary) is the principal
light -gathering surface of a reflectivetelescope .For most of astronomy's history, primary mirrors used to be monolithic blocks of glass or other material, curved to exact shapes and coated with a reflective layer.This worked well, but as telescope diameters began to increase, the primary mirror became also the primary limitation on the telescope size: the mirror had to sustain its own weight and not deform under gravity. The limit was soon reached with the 5-meter
Mount Palomar observatory and a 6-meter in the USSR. For decades, telescope sizes did not increase significantly.Fact|date=July 2007Then, some new technologies were introduced: starting with the MMT, primary mirrors were constructed from small segments, merged (by physical contact or later by optics) into one large primary mirror. While the MMT was a 4.5-meter (now 6.5m), the
Keck telescope s used a 10-meter segmented mirror, and many others are in development at the University of Arizona Steward Observatory Mirror Lab.Fact|date=July 2007Secondly, a thin mirror technology was used together with
active optics : a very thin mirror (in the order ofcentimeters ) is suspended byactuator s in its optimal shape, against the force ofgravity . This allows large non-segmentedmirrors . This technique is used on the VLT and Large Binocular Telescope LBT, and in many other operating or plannedtelescopes .Fact|date=July 2007
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