NASA Reaches Milestone With James Webb Space Telescope Mirror Calibration

Where once there were 18, now there’s just one. The James Webb Space Telescope is currently hanging in space at the Earth-Sun L2 Lagrange point, where it will remain for the remainder of its mission. Before it can get around to doing any science, NASA has to calibrate the instruments and adjust the 18 mirror segments. Initially, every segment created its own image, but NASA reports it has finished calibrating the James Webb Space Telescope’s mirrors so they all focus on the same location.

The James Webb Space Telescope (JWST) is the successor to Hubble, which has been operating now for over 30 years. As the aging observatory runs low on redundant systems, Webb is brand new, and its textbook launch means it could operate for as long as 20 years. Its position about a million miles away allows the telescope’s instruments to remain extremely cold, which is ideal for the mid-infrared observations that will set the JWST apart. Getting this far was a nerve-wracking process for a world that has watched the telescope take shape over the last 20 years — it was folded up inside an Ariane 5 rocket, and there were hundreds of potential points of failure in the origami-like unfolding process.

The last element of the deployment is to get the primary mirror configured, which is why there’s just one image of the star HD 84406 below instead of 18. Unlike Hubble, which had a single parabolic mirror, Webb has a Korsch-style reflector made up of adjustable segments. The first image from Webb showed the guide star 18 times with no rhyme or reason. Then, NASA identified each mirror segment’s position and began nudging them in the right direction. Through this process, the image from each mirror is directed to the same location on the sensor, producing a single stacked image.

This is not the end of the calibration process, though. The segments are now in the right approximate orientation, but close enough only counts in horseshoes and hand grenades. Astronomy, not so much. In its current state, Webb’s primary mirror is acting as 18 small telescopes instead of one large one. Next, the team will begin tweaking each segment, aligning them until the variation is smaller than the wavelength of light. This process is called Coarse Phasing, and it’s the fourth of seven steps to get the observatory’s mirror ready.

So far, everything has gone swimmingly, a welcome departure from Webb’s time on the ground when it seemed like everything that could go wrong did. With all the delays, Webb ended up costing $10 billion. If the rest of the mission goes as well as deployment, it will be more than worth the cost.

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