Deep Field

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Deep Field
29 September 2026 2518 stories tracked

Glossary · Telescopes & Instruments

Segmented mirror

One giant mirror made of many small ones, held in alignment to a fraction of a wavelength.

Past about 8 metres a monolithic mirror sags under its own weight and cannot be cast, shipped or supported. The answer is hexagonal segments on active supports, each nudged continuously so the whole surface behaves as one optical figure — Keck pioneered it with 36 segments, Webb flew 18, and the ELT will have 798. The alignment tolerance is tens of nanometres. Webb's commissioning was largely the process of getting eighteen separate images of one star to become a single point. The segments also solve a problem nobody talks about: a monolith that cracks is a dead telescope, and a segment that cracks is a fortnight's work with a crane. Keck has replaced several without ever stopping observing, and the ELT is designed to recoat a handful of its 798 every week in a plant on site.

Related terms

Lagrange point Five places where two bodies' gravity lets a third sit still relative to both. Adaptive optics Bending a mirror hundreds of times a second to cancel out the atmosphere. Interferometry Combining separate telescopes so they resolve detail like one enormous dish. First light The night a new telescope is pointed at the sky and actually works.

Questions

What is Segmented mirror?

One giant mirror made of many small ones, held in alignment to a fraction of a wavelength.

Why does it matter?

The segments also solve a problem nobody talks about: a monolith that cracks is a dead telescope, and a segment that cracks is a fortnight's work with a crane. Keck has replaced several without ever stopping observing, and the ELT is designed to recoat a handful of its 798 every week in a plant on site.

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