Large Telescope Mirror Technology

Large telescope mirror technology spans the fabrication, polishing, and active support of the primary mirrors that set a telescope's aperture, from monolithic thin meniscus blanks to mosaics of hexagonal segments.

Definition

Large telescope mirror technology is the set of materials, manufacturing, support, and control methods used to produce and maintain primary mirrors several metres or more across that hold an optical figure accurate to a small fraction of the observing wavelength.

Scope

This topic covers casting and figuring of large glass-ceramic blanks, honeycomb and thin-meniscus lightweighting, segmented-mirror architectures with edge sensors and phasing, active optics that maintain figure with actuators, mirror coatings, and the metrology used to test surfaces to fractions of a wavelength.

Core questions

• How are very large mirror blanks cast and figured to optical tolerances?
• When is a segmented mirror preferable to a monolithic one?
• How is mirror figure maintained against gravity, wind, and temperature?
• How are mirror surfaces tested and coated?

Key theories

Active optics and figure control

Thin or segmented mirrors are too flexible to hold their shape passively, so an array of actuators continually corrects low-order figure errors using feedback from wavefront sensing.

Segmentation and phasing

Splitting a large aperture into hexagonal segments eases fabrication and transport, but the segments must be aligned and phased to within a fraction of a wavelength using edge sensors so they act as one optical surface.

Lightweighting strategies

Honeycomb-back, thin-meniscus, and spun-cast borosilicate blanks reduce mass and thermal inertia so mirrors reach ambient temperature quickly and impose lighter loads on the structure.

Clinical relevance

Mirror technology is the principal limit on telescope aperture and therefore on sensitivity and resolution; segmentation and active optics made today's 8-to-10-metre telescopes possible and are central to the extremely large telescopes now under construction.

History

After the 5-metre Hale mirror reached the practical limit of rigid glass blanks, the Keck telescopes pioneered the segmented approach in the 1990s while the European Southern Observatory's New Technology Telescope demonstrated active optics on a thin meniscus. Spin-casting of honeycomb borosilicate blanks and ever-larger segmented arrays now define the field.

Key figures

• Jerry Nelson
• Roger Angel
• Raymond Wilson

Seminal works

• bely2003
• wilson1999

Frequently asked questions

Why are the largest telescopes built from mirror segments instead of one big mirror?

A single mirror much larger than about eight metres becomes extremely heavy, hard to cast without flaws, and impossible to transport. Dividing the aperture into many identical hexagonal segments that are aligned and phased into one surface sidesteps these limits, which is why extremely large telescopes use hundreds of segments.

What is the difference between active optics and adaptive optics?

Active optics corrects slow, large-scale deformations of the telescope itself, such as gravitational sag and thermal change, at rates of seconds or slower. Adaptive optics corrects fast distortions imposed by the atmosphere, hundreds of times per second, using a separate small deformable mirror downstream.