May 23, 2022

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NASA's Webb Telescope Mirror Shatters 'Most Optimistic Forecast' After Final Alignment

NASA’s Webb Telescope Mirror Shatters ‘Most Optimistic Forecast’ After Final Alignment

NASA says the “optical performance of the nascent James Webb Space Telescope (JWST)…remains better than…the most optimistic forecast” after completing the 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 way into deep space on December 25, 2021. The 6.2-ton (roughly 13,600 lb) JWST telescope was nearly half as heavy On liftoff like NASA’s iconic Hubble Space Telescope despite filling an unprecedented origami-like mirror with more than six times the total Hubble collection area. The combination of extreme mass reduction and the unusual complexity required to launch such a large mirror away from Earth with a rocket like Ariane 5 helps partially explain why the Webb Telescope took so long (~18 years) and costly (~$9.7 billion) to design, develop, and build.

However, it was finally released. Ariane 5 did most of the work, sending the telescope on a trajectory – with some help from onboard thrusters – that would point it at the Lagrange point of the Sun and L2 located 1.5 million kilometers (~950,000 miles) from Earth. Perhaps to the greatest relief in the history of space observatories, the highly complex Webb Telescope 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 the much less obvious and much smaller alignments and calibrations. Each of JWST’s 18 main mirror segments slowly but surely hold micrometer by micrometer in position while large areas of the telescope are slowly cooled to ambient temperatures – essential for maximum performance. At the same time, all of Webb’s initial instruments achieved first light and entered the early stages of calibration and commissioning. Only after the instruments painstakingly calibrated, the mirror is perfectly aligned, and the critical instruments are cooled to temperatures as low as -449°F (-267°C), can Webb begin to observe the universe and revolutionize large subsets of space science.

An inside view of the fruits of alignment. (NASA)

The first and most important step – aligning the mirror – is now complete. The alignment process began in February 2022, six weeks after takeoff. First, images were taken with the misaligned mirror to help determine exactly what state it was in. One by one, each of the 18 Webb mirror segments was moved individually to determine which image each mirror was responsible for, allowing the ground controllers to properly focus each mirror view of the target star. In a process known as ‘coarse phases’, once these 18 optical points had been well resolved and attached to a particular mirror segment, the segments were gradually oriented on top of each other to produce a single image.

“Coarse” severely lowers the almost unfathomable precision required to complete the 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 was 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 those Texas-sized portions with each other to an accuracy of about 1.5 inches.”

The product of coarse phases – beautiful but not yet scientifically useful. (NASA)

This is followed by a subtle increment, involving a set of more esoteric operations designed to focus the mirror as fully as possible. The resulting image was then modified to properly align it with the field of view of each of the Webb’s four main science instruments. Finally, some steps of the seven-step alignment process have been completely redesigned or refined to optimize the mirror to the satisfaction of Earthbound creators and potential users.

In the end, the Webb Telescope’s alignment was extraordinarily successful, producing a clearer, cleaner image than the “most optimistic predictions” made by its engineers. NASA says the image is so detailed that it effectively reached the physical resolution of a mirror the size of the Webb Telescope, meaning it would have to violate known laws of physics to resolve any other details.

First JWST images with fully aligned mirror. (NASA)

With the mirror alignment complete, JWST has only one major hurdle left before scientific operations can begin: device operation. Commissioning is a comprehensive statement covering the broad range of calibration, analysis, testing, and optimization required to verify that the four major JWST tools behave as expected and perform the work they are designed to do as accurately and reliably as possible.

At some point, the use of highly complex scientific instruments becomes akin to an art form, and a certain degree of trust must be built between scientists and their hoped-for commercial instruments before they can confidently nail the chisel to marble and begin delving into the universe with unprecedented width and detail. If operation goes 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.

NASA’s Webb Telescope Mirror Shatters ‘Most Optimistic Forecast’ After Final Alignment






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