The James Webb Space Telescope has begun the laborious process of calibration, and has already transmitted its first image. Well, actually, its first 1,560 frames, because that was the number of photos needed to determine the state of the 18 hexagonal mirrors that make up the main mirror.
As already announced, the positioning of these 18 reflectors is not yet perfect. The tremors of its launch and the thermal contractions due to its gradual drop in temperature caused each reflector to point slightly out of step with the rest. They will now move one by one to create a single surface that is as perfect as possible. For now, the Webb does not operate as a single telescope, but rather as 18 different telescopes, each of which sends its own image.
To do this, he first focuses on a bright star that cannot be confused with the others around it. For this task, the rather prosaic name HD84406 was chosen. It is located in Ursa Major, an area that lacks bright stars.
As you might expect, the first pictures taken with the telescope showed not one, but 18 bright spots, that is, the same star seen from 18 mirrors. The telescope took hundreds of photos to determine where HD84406 was, over an area not much larger than the size of the Full Moon. Fortunately, the images were tightly packed together, which says a lot about the quality of the telescope’s structure, given that it withstood the vibrations and acceleration of the launch very well.
The next step was to identify which image corresponded to each reflector. One by one, the technicians tilted each mirror a few nanometers to discover the single point that had moved in a new photo. The identification lasted several hours. Now it is clear who is who.
Based on this information, the next step is to adjust the position of each mirror with microscopic movements. To do this, actuators capable of moving in steps 10,000 times smaller than the diameter of a human hair are used. And they do it without any lubrication and at temperatures of -230°C.
They do this using precision motors and gears, but at first glance they don’t look particularly special. The secret lies in a very rigid H-shaped titanium part and another part that presses on its central bar. Thus, the two side supports flex imperceptibly and this movement – of just a few nanometers – is transmitted to the mirror to put it in place.
It is one of many ingenious solutions that had to be developed for this project, which aims to usher in a new era of astronomy, providing information on the early stages of the universe, how stars were formed and whether or not planets that lie far beyond our solar system could support life.
Another example of problem solving for the project is that all the mirrors have another driver in the middle that allows their curvature to be changed to fit the correct position. This technique of deformable mirrors was developed during the Cold War years to compensate for atmospheric turbulence and thus make it possible to photograph enemy military satellites from the ground. Today, they are used in most major observatories where telescopes can now – sometimes – rival the quality of the images that are sent by the Hubble Space Telescope.
Another of the images that the new telescope has already sent is a selfie in which its own mirror is visible. This aimed to verify that the deployment had gone well and that all the segments were fixed in position. Luckily, one of them was pointed exactly at the star, and its hazy glow can be seen saturating the entire surface. The other 17 reflectors are in the shade, lit only by the dazzling rays of the stars.