NASA tests Webb Telescope to ensure it can be successfully launched into space

Engineers successfully completed the first important optical measurement of the Webb Telescope’s fully assembled primary mirror.

0 November 2, 2016

Engineers working on the James Webb Space Telescope performed a Center of Curvature test, where they took optical measurements of the telescope’s deployed mirror, needed before undergoing several stages of mechanical testing.

NASA Webb telescope

Engineers conduct a “Center of Curvature” test on NASA’s James Webb Space Telescope in the clean room at NASA’s Goddard Space Flight Center, Greenbelt, Maryland. Photo courtesy of NASA/Chris Gunn.

These tests help ensure that the mirror can withstand the extreme space environment and that the conditions don’t alter the shape and alignment of Webb’s primary mirror, which could degrade or ruin its performance.

Making the same optical measurements before and after simulated launch and comparing the results is fundamental to Webb’s development.

“This is the only test of the entire mirror where we can use the same equipment during a before and after test,” said Ritva Keski-Kuha, the test lead and NASA’s Deputy Telescope Manager for Webb at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “This test will show if there are any changes or damages to the optical system.”

In order to conduct the test, optical engineers set up an interferometer, the main device used to measure the shape of Webb’s mirror. Waves of visible light are less than a thousandth of a millimeter long, and optics like Webb’s need to be shaped and aligned even more accurately than this to work correctly. Making measurements of the mirror shape and position by lasers prevents physical contact and damage. So scientists use wavelengths of light to make tiny measurements. By measuring light reflected off the optics using an interferometer, they are able to measure extremely small changes in shape or position.

The Center of Curvature test measures the shape of Webb’s main mirror by comparing light reflected off of it with light from a computer-generated hologram that represents what Webb’s mirror ideally should be. By interfering the beam of light from Webb with the beam from the hologram reference, the interferometer accurately compares the two by measuring the difference to incredible precision.

“Interferometry using a computer-generated hologram is a classic modern optical test used to measure mirrors,” said Keski-Kuha.

During the test, temperature and humidity conditions in the cleanroom were kept incredibly stable to minimize drift in the sensitive optical measurements over time.  Even so, tiny vibrations are ever-present in the cleanroom that cause jitter during measurements, so the interferometer is a ‘high-speed’ one, taking 5,000 ‘frames’ every second, which is a faster rate than the background vibrations themselves. This allows engineers to subtract out jitter and get good, clean results.

Another significant challenge the team faced was the size of the mirror within space telescope.

“We have spent the last four years preparing for this test,” said David Chaney, Webb’s primary mirror metrology lead at Goddard. “The challenges of this test include the large size of the primary mirror, the long radius of curvature, and the background noise. Our test is so sensitive we can measure the vibrations of the mirrors due to people talking in the room.”

The Center of Curvature test will be repeated after the launch environment test to compare the results and ensure that Webb’s optics will work in real world situations.

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