segmented mirror

A segmented mirror is a reflector-telescope mirror formed by aligning smaller mirrors (optical segmentation). This is in contrast to those with a single, i.e., monolithic mirror. The MMT pioneered the use of a segmented primary mirror for large, research telescopes (but its mirror was later replaced by a single mirror when mirror-manufacturing methods improved). Keck Observatory followed, using segmented mirrors in the largest aperture telescopes of the time, and the future extremely large telescopes are to have segmented mirrors. Example telescopes with segmented mirrors:

• Keck
• Hobby-Eberly Telescope (HET)
• Southern African Large Telescope (SALT)
• Gran Telescopio Canarias (GTC)
• James Webb Space Telescope (JWST)
• LAMOST
• European Extremely Large Telescope (E-ELT) (under construction)
• Thirty Meter Telescope (TMT) (under construction)
• Giant Magellan Telescope (GMT) (under construction)
• Overwhelmingly Large Telescope (OWL) (plan set aside)

Substantial non-segmented (monolithic) examples:

• Subaru Telescope
• Hale Telescope
• Large Altazimuth Telescope (BTA-6)
• Gemini Observatory (two telescopes)

Also, the Large Zenith Telescope (LZT) had a (non-segmented) liquid mirror.

Research astronomy requires a high degree of precision in the alignment of the smaller mirrors, and adaptive alignment is often performed under computer control, adjustments made when an observation commences and proceeds, because the weight of the elements of the telescope at a given position, as well as the temperature of the telescope, can impair the alignment.