The James webb It is already a telescope, not a package of metal, plastic sheeting, poles, motors, mirrors, and electronics. In the last hours it finally extended its “wings” of mirrors that until then had been folded on both sides of the main reflector, thus acquiring its six and a half meters in diameter. It was the last critical step before reaching his destination, at Lagrange 2.
Previously the Webb It had completed a series of previous steps: the extension of its solar panel and main antenna. Then, the dreaded deployment and tensioning of the parasol that should protect it from direct sunlight. And on Thursday, the extension of the “tripod” that holds the secondary mirror in position, seven meters in front of the main reflector.
Most of these movements have been done in slow motion, carefully checking each step. Only the extension of the tripod of the secondary mirror took more than half an hour: in that time, the motor shaft that operated one of its pole legs (the other two simply followed the movement) barely made three-quarters of a turn, checking its position tenth of a grade to tenth grade.
All these operations are followed from the control center monitoring the telemetry that arrives from the ship. The Webb he does not carry video cameras to check whether his own deployment has been successful. But what he lacks in sight he makes up for with touch: hundreds of sensors distributed throughout all the on-board mechanisms report the rotation of each motor, the angle of each rod or the setting of each latch.
At one point he considered installing video cameras on board, but the idea was scrapped: deployment is so complicated and has so many critical points that dozens of cameras would have been needed. Its weight and that of the lighting lamps (the mirror and associated instruments are always in the shade) would have reduced the useful load and also because the cameras would have had to be connected to the central body by electrical cables and these give off heat. Minimal, but the engineers did not want to compromise the effectiveness of the sun visor by introducing “hot spots” into it.
The POT has broadcast live all phases of the deployment, illustrated with excellent images. But they are not actual views but an animated, computer generated model. Of course, their movements during each stage of the opening responded exactly to the telemetry signals. Every time something on board was unfolded or a latch engaged, the corresponding sensor sent a signal which, in turn, was reflected on the model.
Each “wing” of the reflector supports three of the 18 mirrors that make up the main reflector. Now, once opened, locked in position and sufficiently cooled, the telescope will begin the slow calibration process.
Despite tight manufacturing tolerances, no one expects the telescope to be perfectly adjusted now. The vibrations of take-off, the opening of its many mechanisms, the expansions and contractions of the structure and, above all, the adjustment of the reflector segments will have to be compensated. In fact, if the Webb focused on a star now, each individual mirror would probably generate its own image out of alignment. It is about adjusting them so that the 18 coincide in one, right where the measuring instruments are.
Each of the 18 mirrors is attached to the structure by four adjustable points: three support it; the fourth, in its center, allows its curvature to be changed very slightly so that between all of them they form an optically perfect concave surface. That implies adjustments of enormous precision. It would take ten thousand individual steps of each of its actuators to move it only the equivalent of a hair’s width. In fact, in fine-tuning mode, the mirrors move slower than the grass grows.
Of course, perfectly calibrating the mirror is a much more complicated process than this simple explanation suggests. The engineers who take care of the telescope have at their disposal several dozen algorithms that allow them to adjust the 18 mirrors one by one. It will take weeks. And the same can be said for the four on-board instruments, which also need to be calibrated. It is not expected to receive the first image of the Webb at least until May.
#James #Webb #telescope #spreading #wings