NASA says the “optical performance of the fledgling James Webb Space Telescope (JWST)…continues to be better than…the most optimistic predictions” after completing the alignment of its record-breaking mirror.
Seven to 14 years behind schedule and over budget by a factor of 2 to 10, an Arianespace Ariane 5 rocket sent the Webb Telescope on its way to deep space on December 25, 2021. Weighing 6.2 tonnes (~13 600 lb), JWST was nearly half the liftoff weight of NASA’s iconic Hubble Space Telescope, despite packing an unprecedented origami-like mirror with more than six times Hubble’s total collection area. The combination of extreme mass reduction and extraordinary complexity required to launch such a large mirror so far from Earth with a rocket like Ariane 5 helps explain in part why the Webb Telescope took so long (~18 years ) and cost so much (~$9.7 billion). ) to design, develop and build.
Nevertheless, the launch eventually did. Ariane 5 did most of the work, sending the telescope on a trajectory that, with the help of its onboard thrusters, would guide it to the Sun-Earth Lagrange point L2 located about 1.5 million kilometers away (~950 000 miles) from Earth. In perhaps the greatest relief in the history of space observatories, the extremely complex deployment process of the Webb Telescope was then 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, almost all of JWST’s work has focused on the less visible and much smaller alignment and calibration processes. Each of JWST’s 18 main mirror segments slowly but surely pushed micrometer by micrometer into position as broad swathes of the telescope slowly cooled to ambient temperatures – essential for maximum performance. Simultaneously, all of Webb’s primary instruments reached first light and entered the early stages of calibration and commissioning. Only after the instruments have been carefully calibrated, the mirror is perfectly aligned, and crucial hardware is cooled to temperatures as low as -449°F (-267°C) can Webb begin to observe. the universe and revolutionize vast subsets of space science.
The first and most important step – aligning the mirrors – is now complete. The alignment process began in February 2022, six weeks after liftoff. First, images were captured with the mirror unaligned to help determine exactly what state it was in. One by one, each of Webb’s 18 mirror segments was moved individually to determine which image each mirror was responsible for, which then allowed the ground controllers to focus correctly. the view from each mirror of a target star. In a process known as “coarse phasing”, once these 18 points of light were well resolved and linked to a specific mirror segment, the segments were gradually directed at each other to produce a single image.
“Coarse” greatly underestimates the almost unfathomable accuracy required to complete the stage. To reach its full potential, each of the Webb telescope’s mirror segments must be aligned within 50 nanometers one another. According to NASA, “If the main Webb mirror was the size of the United States, each segment would be the size of Texas, and the team would need to align the height of these Texas-sized segments with each other. with an accuracy of about 1.5 inches.
Fine phasing followed, involving an even more esoteric set of processes aimed at focusing the mirror as perfectly as possible. The resulting image was then edited to align it correctly with the field of view of each of the Webb Telescope’s four main science instruments. Finally, certain steps in the seven-step alignment process have been redone or refined to fully optimize the mirror to the liking of its Earthbound creators and potential users.
In the end, the alignment of the Webb telescope was extraordinarily successful, producing a sharper and sharper image than even the “most optimistic predictions” made by its engineers. NASA says the image is so detailed that it has effectively reached the physical resolution limit for a Webb telescope-sized mirror, meaning it would have to violate known laws of physics to resolve more detail. .
With the alignment of the mirrors complete, the JWST has only one hurdle to clear before science operations can begin: commissioning the instruments. Commissioning is a catch-all phrase that covers a wide range of calibrations, analyses, experiments, and optimizations necessary to verify that JWST’s four main instruments perform as intended and do the job for which they are intended. they have been designed as accurately and reliably as possible.
At some point, the use of extraordinarily complex scientific instruments becomes more like an art form, and some degree of trust must be established between scientists and their hopeful tools of the trade before they can confidently set the chisel on the marble and begin to delve into the universe with unprecedented breadth and detail. If commissioning goes as smoothly as deployment and alignment, the JWST team could be ready to capture and share the telescope’s first actionable observations of the cosmos as early as July 2022.