Almost there: James Webb Space Telescope frees its mirrors and prepares for insertion

Freed of launch restraints, mirror segments can waggle at will

NASA scientists have deployed mirrors on the James Webb Space Telescope ahead of a critical thruster firing on Monday.

With less than 50,000km to go until the spacecraft reaches its L2 orbit, the segments that make up the primary mirror of the James Webb Space Telescope (JWST) are ready for alignment. The team carefully moved all 132 actuators lurking on the back of the primary mirror segments and secondary mirror, driving the former 12.5mm away from the telescope structure.

Now clear of their launch restraints, each segment has enough space to be adjusted during the upcoming alignment process.

In addition, 18 radius of curvature (ROC) actuators were moved from launch position. Together, the ROC actuator will shape the primary mirror into its initial parabolic form.

With the more than a million revolutions of the motors complete, attention can turn to adjustments in the micron and nanometer ranges in order get the telescope properly aligned – a process expected to take around three months.

The completion of the deployment comes as engineers prepare to fire the JWST's thrusters in order to enter an orbit around the second Sun-Earth Lagrange point. The nature of the burn is an interesting one, since an intentional under-burn was made when the Ariane 5 launch vehicle sent the JWST on its way. The thrusters will be used to make up the difference.

Had the JWST received too much thrust from its launcher, it would have had to turn around and thrust back toward Earth, thus exposing bits to the Sun that would have caused an abrupt end to the science mission.

As it was, the performance of the Ariane 5 was so precise that engineers estimate the JWST will end up with far more than the baseline estimate of propellant (required for station-keeping as well as the insertion firing), thus potentially extending the duration the observatory can keep operating. ®

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