James Webb Telescope: after the sun visor, make way for mirrors

James Webb Telescope: after the sun visor, make way for mirrors


This Tuesday, January 4, NASA operators successfully closed the process of releasing the sun visor of the James Webb Telescope. It was the most complex and delicate operation of the observatory’s long deployment phase. The Mission Team can now move on to the next important item on their to-do list: deploying the secondary and primary mirrors.

The James Webb Telescope continues its process of millimeter deployment into space. On Monday and Tuesday, mission controllers began and closed the deployment of its massive sunshade, a five-layer shield the size of a tennis court. According to NASA, each sheet is about as thin as a human hair and covered with reflective metal, reducing exposure to heat from the Sun, Earth and Moon.

Now, make way for mirrors
The main mirror of the James Webb Telescope, the largest ever built by NASA (6.5 m in diameter) consists of eighteen hexagonal shaped segments constructed from beryllium, a very light metal. Each of these structures is lined with gold, the material offering the highest reflectivity over a very wide band of wavelengths.

This mirror is broken down into three parts: a central platform and two smaller side “wings”. These were folded up before launch so that the assembly could fit inside the Ariane 5 rocket fairing. Deploying the main mirror will therefore consist of deploying these two wings.

The secondary mirror represents the second surface, after the primary mirror, that the light will strike on its way to the four scientific instruments of the telescope. The 0.74m structure sits at the end of several spiers that need to be extended. The secondary mirror will be deployed first. The whole process is set to begin in earnest this weekend and take about ten days.

Align each segment
If all goes as planned, the telescope will then glide into orbit around the point of Lagrange 2 about 29 days after launch, on January 23. At this point, the mission team will still need to precisely align the eighteen primary mirror segments so that they act as a single light-collecting surface.

To do this, the mission controllers will turn on the telescope’s near infrared camera (NIRCam). This instrument is designed to detect incoming light distortions with extreme precision. From then on, 126 actuators will take care of positioning, bending or subtly flexing each segment according to the data transmitted by the instrument.

The following weeks will involve final calibrations and the completion of the commissioning process. Scientific operations should finally begin this summer and last between five and ten years.