GMT's Primary Mirror

mirror segments

The principal measure of the power of an optical/IR telescope is the diameter of its primary aperture.

preparing cast

Larger apertures translate into both greater light collecting area and, potentially, higher angular resolution. GMT’s primary mirror comprises seven segments that work together like a single mirror with the resolving power of a telescope 24 .5 meters (over 80 feet) in diameter. Each of GMT’s seven mirror segments is 8.4 meters in diameter. The limitation of the size of a single mirror segment is related to the technology available to manufacture and transport such a mirror. GMT’s 8.4-meter mirror segments are being developed at the University of Arizona’s Steward Observatory Mirror Lab (SOML).

The mirrors are made of low expansion glass molded into a light-weight honeycomb structure. The mirror segments are ground and polished to a precise optical prescription. The final polished surface departs from the desired shape by no more than 1/20 of a wavelength of green light, or approximately 25 nanometers. After polishing, the surface is coated with a thin layer of aluminum to achieve maximum reflectivity.

One of the mirror segments is mounted at the center axis of the telescope. The other six mirrors are mounted surrounding the center mirror segment. Each mirror segment is mounted into its own “cell,” a complex active support system that keeps the mirror in the proper position relative to the other segments at all times.


The most challenging aspect of making the GMT’s mirror segments arises from the asymmetric shape of the six outer segments. These mirrors have a steeply curved shape similar to that of a potato chip. The peak departure from a symmetric mirror is 14 mm at the edge, amounting to 28,000 waves of green light. A new suite of test instruments and procedures were developed to test these mirrors.

The first mirror was successfully cast at the SOML and polishing is nearly complete.

How are the GMT Mirror Segments Made?

Video: Inside
Mirror Lab