NASA said the “optical performance of the nascent James Webb Space Telescope (JWST)…stayed better than…most optimistic estimates” after completing a record-breaking mirror alignment.
Between 7 and 14 years behind schedule and budget by a factor of 2 to 10, the Arianespace Ariane 5 rocket sent the Webb Telescope on its journey into space on December 25, 2021. The 6.2 ton (roughly 13,600 lb) JWST telescope is almost half the weight. As it lifts off like NASA’s iconic Hubble Space Telescope though it fills an unprecedented origami-like mirror with more than six times the total Hubble collection area. The combination of the extreme reduction in mass and the unusual complexity required to launch such a large mirror from Earth on a rocket like the Ariane 5 helps partly explain why the Webb Telescope took so long (~18 years) and was expensive (~$9.7 billion) to design. , develop, and build.
However, it was finally released. Ariane 5 does most of the work, sending the telescope on a trajectory – with help from its onboard thrusters – that will point it to the Sun’s Lagrange points and L2, which are 1.5 million kilometers (~950,000 miles) from Earth. Perhaps most relieved in the history of space observatories, the Webb Telescope’s incredibly complex deployment process was completed without a single major problem. 30 days after liftoff, the telescope – fully deployed – reached its operational orbit.
Over the past four months, by comparison, nearly all of JWST’s work has focused on much less obvious and much smaller alignment and calibration. Each of JWST’s 18 main mirror segments slowly but steadily holds micrometer by micrometer in position while the large area of the telescope is slowly cooled to ambient temperature – essential for maximum performance. At the same time, all of Webb’s early instruments reached their first light and entered the initial stages of calibration and commissioning. Only after the instrument was painstakingly calibrated, the mirrors perfectly aligned, and the critical instrument cooled to temperatures as low as -449°F (-267°C) could Webb begin observing the universe and revolutionizing a large subset of space science.
The first and most important step – aligning the mirrors – is now complete. The alignment process begins in February 2022, six weeks after liftoff. First, the image is taken with an misaligned mirror to help pinpoint the situation. One by one, each of Webb’s 18 mirror segments were moved individually to determine which image each mirror was responsible for, allowing the earth controller to precisely focus each mirrored view of the target star. In a process known as the ‘coarse phase’, after these 18 optical points have been fine-tuned and attached to a specific mirror segment, the segments are gradually oriented on top of each other to produce a single image.
“Coarse” greatly decreases the near-unpredictable precision required to complete a move. To reach its full potential, each Webb Telescope mirror segment must be aligned within 50 nm from each other. According to NASA, “If the initial Webb mirror were the size of the United States, then each fragment would be the size of Texas, and the team would need to align the heights of the Texas-sized sections with each other to an accuracy of about 1.5 inches.”
This was followed by a subtle upgrade, which involved a series of more esoteric operations designed to focus the mirror as much as possible. The resulting images were then modified to align them with the respective fields of view of Webb’s four main science instruments. Finally, several steps of the seven-step alignment process have been completely redesigned or refined to optimize the mirror for the satisfaction of Earthbound creators and potential users.
In the end, the alignment of the Webb Telescope was a resounding success, producing a clearer and purer image than the “most optimistic predictions” made by its engineers. NASA says the image is so detailed that it effectively reaches the physical resolution of a mirror the size of the Webb Telescope, meaning the image would have to violate known laws of physics to resolve other details.
With mirror alignment complete, JWST has only one major hurdle remaining before scientific operations can begin: device operation. Commissioning is a comprehensive statement covering the various calibrations, analyses, tests, and optimizations required to verify that the four major JWST tools are behaving as expected and doing the job they were designed to do as accurately and reliably as possible.
At some point, the use of highly complex scientific instruments became akin to an art form, and a certain level of trust had to be built between scientists and the commercial instruments they hoped for before they could confidently nail chisels to marble and start digging inside. a universe of unprecedented width and detail. If operations go as smoothly as deployment and alignment, the JWST team may be ready to capture and share the first actionable telescope observations of the universe as early as July 2022.
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