The telescope james webb has begun its laborious calibration process and has transmitted its first photograph. Well, actually, its first 1,560, because that has been the number of images needed to know the state of the 18 hexagonal mirrors that make up the main mirror.
As was known in advance, the adjustment of the 18 reflectors is not perfect. Launch shocks and thermal contractions due to its progressive cooling have caused each reflector to point ever so slightly out of alignment with the others. It is about moving them one by one to recompose together a single surface as perfect as possible (a hyperboloid, to be exact). For now, the webb It does not behave like a single telescope, but like 18 different ones, each of which sends its own image.
How do you fit such a puzzle? First, looking for a guide star that is bright, but isolated so that there are no others around it that could confuse the process. One has been chosen that responds to the not at all poetic name of HD84406. “HD”, by the way, are the initials of the catalog of Henry Draper, an amateur astronomer who in 1872 began to collect photographs of spectra; today, it has grown above 225,000. The one that occupies us is one more, located in the car of the Big Dipper, an area especially devoid of bright stars.
The first photos taken with the telescope unsurprisingly showed not one, but 18 bright spots. The same star seen by the 18 mirrors. Not being very sure of the orientation of each one, hundreds of photos had to be taken to sweep the entire area where HD84406 could be found. An area no larger than the disk of the full Moon. Luckily, they turned out to be very close together. That little dispersion says a lot about the quality of the structure that supports them, which resisted the vibrations and acceleration of the launch very well.
The next step was to identify which image corresponded to which reflector. One by one, the technicians tilted each mirror a few nanometers to discover, in a new photo, the only spot that had moved. Identification took a few hours. Now it is known without ambiguity which is which.
With that information, the next step will be to adjust the position of each mirror in microscopic displacements. To do this, mechanisms capable of moving in steps 10,000 times smaller than the diameter of a hair are used. And this, without lubricant and at temperatures of 230 ºC below zero. How can such tolerances be achieved?
Surprising as it may seem, the actuators have very precise motors and gears, but at first glance they are nothing special. The secret is in a very rigid piece of titanium in the shape of an “H” and a punch that presses on its central bar. As a result, the two lateral uprights bend imperceptibly and it is this movement – a few nanometers – that is transmitted to the mirror to orient it.
It is one of many ingenious solutions that had to be developed for this project. But not the only one. All the mirrors have another actuator in the center that allows them to change their curvature very slightly to finish adapting it to the correct position. The deformable mirror technique was developed during the Cold War years to compensate for atmospheric turbulence and to be able to photograph the appearance of enemy military satellites from the ground. Today it is used in most of the large observatories where their telescopes can already compete —sometimes— with the quality of the images sent by the Hubble. In the case of webbit is used simply to adjust its shape so that it responds to the specifications.
Another of the images that the new telescope has sent is a selfie in which you can see your own mirror. It was about checking that the deployment had gone well and that all the segments were locked in position. By pure chance, one of them was exactly towards the star, and its brilliance, out of focus, saturates its entire surface; the other 17 reflectors are in shadow, lit only glancingly by the glare of the stars.
You can follow MATTER in Facebook, Twitter and Instagramor sign up here to receive our weekly newsletter.
Exclusive content for subscribers
read without limits
#James #Webb #opens #eyes #time