NASA Completes JWST Cryogenic Testing

NASA researchers have completed cryogenic testing on the science instruments module for the James Webb Space Telescope (JWST). The module contains JWST’s science cameras and spectrographs. It had been undergoing four months of monitoring inside a giant thermal vacuum chamber called the Space Environment Simulator (SES) at NASA’s Goddard Space Flight Center in Greenbelt MD. The 40-foot-tall, 27-foot-diameter cylindrical chamber duplicates the extreme temperatures and vacuum of space, eliminating almost all of the air with vacuum pumps and dropping the temperature using liquid nitrogen and gaseous helium.

The testing is critical because these instruments must function in space at 40K (-387°F). “We needed to test these instruments against the cold because one of the more difficult things on this project is that we are operating at very cold temperatures,” says Begoña Vila, NASA’s cryogenic test lead for the ISIM at NASA Goddard. The ISIM, or Integrated Science Instrument Module, is one of three major elements that comprise the JWST flight system. “We needed to make sure everything moves and behaves the way we expect them to in space. Everything has to be very precisely aligned for the cameras to take their measurements at those cold temperatures which they are optimized for.”

The module that was tested consists of the mid-infrared instrument (MIRI), jointly developed by a nationally funded European Consortium under the auspices of the European Space Agency (ESA) and the Jet Propulsion Laboratory; a near infrared spectrometer (NIRSpec), jointly developed by Airbus for ESA and the US; the Fine Guidance Sensor/Near-InfraRed Imager and Slitless Spectrograph, provided by the Canadian Space Agency and developed by COM DEV International, Cambridge, Ontario, Canada; and the Near Infrared Camera (NIRCam), built by a team at the University of Arizona and Lockheed Martin’s Advanced Technology Center.

Shortly after instrument testing, researchers installed the last mirrors of JWST’s optical path into the telescope, officially making the telescope optically complete. “Optical completeness means that all of the telescope mirrors have been installed,” says Lee Feinberg, the JWST optical telescope element manager at Goddard. “We can now say ‘we have a telescope.’ It’s a huge milestone many years in the making.”

Once launched into space, JWST will capture faint light from the very first objects that illuminated the universe after the Big Bang. To make observations of galaxies and stars from that far away, the telescope has a unique set of mirrors: a 25 square meter (~269 square feet) primary mirror consisting of 18 hexagonal concave segments, a secondary rounded, convex mirror, a tertiary concave mirror and a movable turning flat mirror called the fine steering mirror.

JWST’s primary mirror segments and secondary mirror are made of beryllium, selected for its stiffness, light weight and stability at cryogenic temperatures. Bare beryllium is not very reflective of near-infrared light so each mirror is coated with about 0.12 ounces of gold to enable it to efficiently reflect infrared light (which is what the JWST’s cameras see).

The anchor of the optical system is the last (third) set of mirrors—the tertiary mirror and the fine steering mirror. These two mirrors are located inside the Aft Optics Subsystem (AOS), a phone booth-sized beryllium structure surrounded in black covering. It is located right in the center of the primary mirror and about half of it sticks up above the primary mirror.

After incoming light hits the expansive primary mirror, it is directed onto the small circular secondary mirror that reflects it back in the direction of the primary mirror and into the AOS. Inside the back end of the AOS is the tertiary mirror where light bounces forward to the fine steering mirror at the ‘front’ of the AOS, which in turn reflects the light out the back of the AOS to a focus behind the primary mirror for the scientific instruments.

The mirrors were built by Ball Aerospace & Technologies Corporation in Boulder CO. Ball is the principal subcontractor to Northrop Grumman for the optical technology and optical system design. The installation of the mirrors onto the telescope structure is performed by Harris Corporation, a subcontractor to Northrop Grumman. Harris Corporation leads integration and testing for the telescope.

“The completion of these major milestones represent huge achievements for NASA and our industry, European, Canadian and academic partners,” says Bill Ochs, JWST project manager at Goddard. “ISIM and the telescope only reached these milestones because of the passion, dedication and imagination of an outstanding group of individuals. The next major step is assembling the instrument module and the telescope together to complete the entire cold section of the Webb observatory.”