Application
Because current monolithic mirrors cannot be constructed larger than about eight meters in diameter, the use of segmented mirrors is a key component of current large-aperture telescopes. This is because of the technological limit of a primary mirror made of a single rigid piece of glass. Using a monolithic mirror much larger than 5 meters is prohibitively expensive due to the cost of both the mirror, and the massive structure needed to support it. A mirror beyond that size would also sag slightly under its own weight as the telescope was rotated to different positions, changing the precision shape of the surface. Segments are also easier to fabricate, transport, install, and maintain over very large monolithic mirrors.
Segmented mirrors do have the drawback that each segment may require some precise asymmetrical shape, and rely on a complicated computer-controlled mounting system. All of the segments also cause diffraction effects in the final image.
Read more about this topic: Segmented Mirror
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