Engineers unfolded the James Webb Space Telescope's spine in the world's largest clean room in preparation for decking it with mirrors like a giant, glittering Christmas tree.

Pathfinder deployed in the Goddard clean room. Image credit: NASA/Chris Gunn

The James Webb Space Telescope is part of the next generation of space-based telescopes. This optical telescope will be a replacement and upgrade for the aging Hubble Space Telescope.

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Pathfinder under construction with secondary mirror boom fully extended. When constructed, this will be tilted upright. Image credit: Northrop Grumman/Alex Evers

Pathfinder under construction in California. Image credit: Northrop Grumman/Alex Evers

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The telescope's spine, or backplane, is named Pathfinder. It is the primary support structure for the spacecraft. All 18 segments that will compose the 21-foot diameter primary mirror for the telescope, instruments, thermal control systems, and any other hardware for the mission will be mounted to Pathfinder. The primary mirror segments will lie on the lattice, while the secondary mirror will sit at the ends of long booms.

Pathfinder needed to be flown from Redondo Beach, California to Greenbelt, Maryland for testing in Goddard's clean room. Image credit: NASA/Desiree Stover

Pathfinder was assembled and tested at Northrop Grumman in Redondo Beach, California, then flown across the country in a C-5 aircraft to the U.S. Air Force's Joint Base Andrews in Maryland. From there, it was unloaded and trucked to NASA's Goddard Space Flight Center in Greenbelt, Maryland for testing in the world's largest clean room.

Time lapse assembly of the Pathfinder backplane in the Goddard clean room.

The backplane currently sprawled inside the clean room is a non-flight copy. Precise mirror placement is essential for making an effective telescope without strange optical distortions, so the engineers will be practicing how to hang mirrors on the deployed non-flight copy before tackling assembly of the real telescope. The copy isn't a perfect match: while the copy contains only the central spine, the real Pathfinder consists of a central strut and two side struts.

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Before installing mirrors, Pathfinder needed to be over-deployed: stretching the struts to maximum extension. The struts were even secured to the walls, providing a stable base for the mirrors while giving engineers plenty of room to work.

The space telescope's Pathfinder unit deployed in the Goddard clean room. Image credit: NASA/Chris Gunn

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After testing is completed at Goddard, Pathfinder will make yet another journey to NASA's Johnson Space Center in Houston, Texas. Once in Texas, it will undergo more engineering tests, dropping to bitter cold temperatures in a cryotesting chamber designed to simulate the temperatures in space.

You can watch construction and testing with this pair of webcams.