Image Caption: Technicians and scientists check out one of the Webb telescope's first two flight mirrors in the clean room at NASA's Goddard Space Flight Center in Greenbelt, Md. Credit: NASA/Chris Gunn

Rob Gutro, NASA

When you think of a mirror, there really isnt that much needed to describe it, but when you look at a mirror that will fly aboard NASAs next-generation James Webb Space Telescope, theres a lot to the anatomy of a mirror.

NASAs Webb telescope includes a primary, secondary and tertiary mirror. Although the relatively small secondary and tertiary mirrors are unique, its the expansive primary mirror that has the most complicated anatomy with a number of components operating together to make the telescope work.

The mirrors were built by Ball Aerospace & Technologies Corp., Boulder, Colo. Ball is the principal subcontractor to Northrop Grumman for the optical technology and lightweight mirror system. Ball Aerospace also developed the secondary mirror, tertiary mirror and fine-steering mirror.

The raw power of any telescope is determined by the size of its main optic the bigger the first or primary optic, the betterand in the case of large telescopes, the optic is a mirror. Webbs primary mirror measures 6.5 meters (21 feet, 4 inches) across, and although thats respectable by ground-based telescope standards, it is absolutely huge for a space telescope. A mirror this large and in space is needed to capture the light from the most distant galaxies and stars in the universe, but it would too big to launch into space if it were one single piece, so thats why Webbs is composed of 18 smaller lightweight segments that can be folded up to fit into the nosecone of a rocket. Each of Webbs 18 hexagonal-shaped primary mirror segments measures just over 1.3 meters (4.2 feet) across, and weighs approximately 40 kilograms (88 pounds). All of the 18 primary mirror segment assemblies that will fly aboard NASAs James Webb Space Telescope have already arrived at NASAs Goddard Space Flight Center in Greenbelt, Md.

Each of the 18 mirror segments is not just a mirror but is a complex assembly of technologies that allows all of them to work together as one. Each mirror has an anatomy of many parts, from the reflective gold-coated Beryllium substrate or layer, down to a Beryllium structure of whiffles and a Delta frame, plus precision actuators to position and shape the mirror, mounted on Backplane Interface Flexures.

The complexity of the mirror assemblies comes from the fact that they are designed to be very lightweight, work at cryogenic temperatures below -400F, survive launch vibration and forces, be align-able on-orbit via actuators, and then stay aligned for up to two weeks as though they are a single large mirror, said Lee Feinberg, NASA Optical Telescope Element Manager for the James Webb Space Telescope at the Goddard Space Flight Center in Greenbelt, Maryland.

Beryllium Mirror Substrate (Smooth mirror surface)

The Beryllium Mirror Substrate is the part of each mirror segment that acts as a mirror in the classic sense. Each substrate is nearly 2 inches thick with a highly-polished and exquisitely smooth front reflective side and a back side that is precision machined into a sort of egg crate-looking structure to make it lighter weight than it would be if solid. The reflective surface is polished to an average roughness of only 20 nanometers (i.e., 20 billionths of a meter) and coated with a microscopically thin layer of pure gold to maximize its ability to reflect infrared light. Beryllium is the material of choice because is it extremely stiff and lightweight, and it behaves very stably and predictably at Webbs extremely cold operating temperatures.

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Amazing Anatomy Of The James Webb Space Telescope Mirrors