Engineers and technicians working on the James Webb Space Telescope (JWST) have successfully completed the first important optical measurement of the fully assembled primary mirror, called a Centre of Curvature test. The JWST — a joint project of NASA, the European Space Agency and the Canadian Space Agency — will possess a 6.5m primary mirrot, the largest of any space telescope.

The mirror comprises 18 separate segments made of ultra-lightweight beryllium that unfold and adjust to shape after launch.

Taking a “before” optical measurement of the telescope’s deployed mirror is crucial before the telescope goes into several stages of rigorous mechanical testing. These tests will simulate the violent sound and vibration environments the telescope will experience inside the rocket on its way into space and which could ptentially alter the shape and alignment of the mirror. 

By measuring light reflected off the optics using an interferometer, engineers are able to measure extremely small changes in shape or position.  The Centre of Curvature test measures the shape of Webb’s main mirror by comparing reflected light with light from a computer-generated hologram that represents what Webb’s mirror ideally should be.

After the measurements come back from the interferometer, the team will analyze the data to make sure the mirrors are aligned perfectly before the launch environment tests. The Centre of Curvature test will be repeated after the launch environment testing and the results compared to confirm that Webb’s optics will work after their launch into space.

The telescope is scheduled for launch on an Ariane 5 rocket from French Guiana in October 2018.