Webb is approaching its destination with critical mirror adjustment on tap – Spaceflight now


The James Webb Space Telescope with five-layer sunshade and optical elements fully unfolded. Credit: NASA

Thirty days out of Earth, the James Webb Space Telescope will slide into its orbit a million miles away on Monday, an ideal place to scan the sky in search of dim infrared light from the first generation of stars and galaxies.

But getting there – and successfully installing a giant awning, mirrors and other pendants along the way – was just half the fun.

Scientists and engineers now have to turn the $ 10 billion Web into a working telescope that precisely adjusts its 18 primary mirror segments to work together as a single 21.3-foot-wide mirror, by far the largest ever been launched.

Earlier this week, the mission operations team remotely completed a multi-day process to raise each segment, and the telescope’s 2.4-foot-wide secondary mirror, half an inch out of the launch pads that held them firmly in place during the observatory’s Christmas climb to space on top of a European Ariane 5 rocket.

Now fully implemented, the 18 segments are currently adjusted within about a millimeter or so. In order for the telescope to achieve a razor-sharp focus, this adjustment must be fine-tuned to within 1 / 10,000 of the width of a human hair using multiple actuators to tilt and even change the shape of a segment if required.

“Our primary mirror is segmented, and these segments need to be adjusted to a fraction of a wavelength of light,” said Lee Feinberg, optical telescopic element leader at NASA’s Goddard Space Flight Center. “We are not talking microns, we are talking about a fraction of a wavelength. That is what is difficult about Webb.”

Once adjusted and its instruments calibrated, Webb will be 100 times more powerful than Hubble, NASA says, so sensitive to infrared light that it could detect the faint heat of a bumble bee as far away as the moon.

Credit: On Monday, Webb will glide in orbit around Lagrange Point 2 nearly a million miles away, combining the gravity of the sun and the earth to form a stability pocket where spacecraft can remain in place with minimal amounts of fuel. Credit: NASA

Each mirror segment was ground to a recipe that takes into account the deforming effects of gravity during their fabrication on Earth and their expected shrinkage in the ultra-low temperatures of space. They were so precisely designed that if blown up in the air the size of the United States, the 14,000-foot-high Rocky Mountains would be less than 2 inches high.

But if Webb were aiming for a shining star today, the result would be 18 separate images “and they’re going to look awful, they’ll be very blurry,” Feinberg said in an interview, “because the primary mirror segments are not in line yet.”

It’s the next big obstacle for the Webb team, which maps and tilts each segment in small intervals, merging these 18 images to form a single precisely focused point of light. It is a multi-step iterative process that is expected to take several months to complete.

But first, the telescope must orbit Lagrange Point 2, 930,000 miles from Earth, where the gravity of the Sun and Earth combine to form a stability pocket that allows spacecraft to remain in place with a minimum of fuel consumption.

It is also a point where Webb’s tennis-sized solar sails can work to maximum advantage and block heat from the sun, earth, moon and even hot interplanetary dust that would otherwise flood the telescope’s sensitive infrared detectors.

As of Saturday, the mirror segments had cooled down to around minus 340 Fahrenheit, well on its way to an operating temperature of around minus 390, or slightly less than 40 degrees above absolute zero.

While the cooling process continues, a 4-minute 58-second course correction of thruster firing is scheduled for Monday at 7 p.m. .

If all goes well, the telescope will remain in the six-month orbit for the rest of its operational life, firing its station-holding thruster at regular intervals to remain at the station.

With the orbit deployment burned behind them, engineers will push forward with mirror alignment, one of the most complex aspects of Webb’s already complicated implementation.

Each 4.3-foot-wide hexagonal primary mirror segment has six mechanical actuators in a “hexapod” arrangement on the rear, allowing movement in six directions. A seventh actuator can push or pull in the center of a segment to distort its curvature if necessary.

Web’s primary mirror consists of 18 hexagonal gold-plated beryllium segments that must be adjusted within a small fraction of the width of a human hair to achieve a sharp focus. This image shows the mirror in preparation for launch with the telescope’s secondary mirror folded away for flight. Credit: NASA

After Webb’s Near Infrared Camera, or NIRCam, has cooled down to its operating temperature, Webb will be aimed at a shining star so that the instrument can map the reflections from all 18 segments and create a mosaic showing their relative size and position.

The mirror segments will then be adjusted one at a time using one actuator and then another, to correct each one. Additional mosaics will be made as the process continues, and depending on the results, the alignment process may need to be repeated.

“The great thing is to get the 18 primary mirror segments to point in a similar way so that their images are about the same size,” Feinberg said. “Some of them can be very unfocused, so you can get a big spot (blurred constellation) on segment 5 and a small spot on segment 3.”

The goal is to tilt the segments as needed to minimize the size of the unfocused images and then move the multiple reflections to the same point in the center of the telescope’s optical axis, all stacked on top of each other to produce a single beam of sharp focused light.

“At the top level, think of it as 18 separate telescopes aligned to about the same level,” Feinberg said. “And then we will overlap 18 seats on top of each other. We call this image stacking. It is a process of tilting the primary mirror segments so that the images fall on top of each other. ”

The key, he said, is “you really need very good control over these actuators, very precise slopes, because we need these 18 places to overlap each other very well.”

Any given segment can lose one of its six rocker actuators without impact. Even the loss of a center actuator can be compensated to some extent by moving the segment slightly up or down.

But exhaustive tests on the ground showed that the high-tech actuators are extremely reliable. The procedures were tested before launch using a sub-scale model of the telescope, and Feinberg said he is confident the alignment process will work as planned.

“When do we get an image of a star that is phased (properly stacked and focused)? I think it will be sometime in March, maybe late March,” he said.

“But then the next question is, when will we have the telescope fully aligned, including the secondary mirror, optimized for all four instruments? The original plan had us to achieve it all four months inside the mission. So that would be the end. of April. “

It will still not be enough for scientific observations to begin.

Once the optical system is adjusted, the team will focus on testing and calibrating the NIRCam, a combination camera and spectrograph, and the telescope’s three other spectrographic instruments, one of which includes the fine control sensor needed to keep Webb locked to the target.

That process will take another two months or so to complete. Only then will focused “first light” images be released to the public.

“We want to make sure that the first images that the world sees, that humanity sees, do justice to this $ 10 billion telescope and are not images of, you know, hey look, a star,” said Jane Rigby, Webb operations project researcher at Goddard.

“So we’re planning a series of ‘wow’ images to be released at the end of commissioning when we start normal scientific operations designed to show what this telescope can do … and really hit all the socks of.”

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