Webb closes in on destination with critical mirror alignment at hand – Spaceflight Now



The James Webb Space Telescope with its five-layer sunshield and fully extended optical elements. Credit: NASA

Thirty days from Earth, the James Webb Space Telescope will slip into its million-mile parking orbit on Monday, an ideal place to scan the skies for faint infrared light from the first generation of stars and galaxies.

But getting there – and successfully deploying a giant sun visor, mirrors and other appendages along the way – was only half the fun.

Scientists and engineers now need to turn the $10 billion Webb into a working telescope, precisely aligning its 18 primary mirror segments so they work together as a single 21.3-foot-wide mirror, far the largest ever launched.

Earlier this week, the mission operations team remotely completed a multi-day process to elevate each segment, and the telescope’s 2.4-foot-wide secondary mirror, half an inch from the launch locks. that held them firmly in place during the Observatory’s Christmas Day. ride into space atop a European Ariane 5 rocket.

Now fully deployed, all 18 segments are currently aligned to within about a millimeter. For the telescope to achieve razor-sharp focus, this alignment must be fine-tuned to less than 1/10,000th the width of a human hair using multiple actuators to tilt and even change the shape of a segment if necessary.

“Our primary mirror is segmented, and those segments need to be aligned over a fraction of a wavelength of light,” said Lee Feinberg, optical telescope elements manager at NASA’s Goddard Space Flight Center. “We’re not talking about microns, we’re talking about a fraction of a wavelength. That’s the tricky thing about Webb.

Once aligned and its instruments calibrated, Webb will be 100 times more powerful than Hubble, according to NASA, so sensitive to infrared light that it could detect faint heat from a bumblebee as far away as the moon.

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

Each mirror segment has been ground to a prescription that takes into account the distorting effects of gravity during their manufacture on Earth and their expected shrinkage in the ultra-low temperatures of space. They were calculated with such precision that if any of them were enlarged to 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 bright star today, the result would be 18 separate images “and they’re going to look terrible, they’re going to be very blurry,” Feinberg said in an interview, “because the primary mirror segments don’t are not yet aligned.

This is the next major hurdle for the Webb team, mapping and then tilting each segment in small increments, merging those 18 images to form a single, exactly focused point of light. This is an iterative, multi-step process that is expected to take several months.

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 pocket of stability that allows the spacecraft to stay in place with minimal fuel expenditure.

It’s also a point where Webb’s tennis-court-sized sunshade can work to its fullest, blocking heat from the sun, Earth, moon, and even hot interplanetary dust that would otherwise , would overwhelm the telescope’s sensitive infrared detectors.

On Saturday, the mirror segments had cooled to about minus 340 degrees Fahrenheit, well on the way to an operating temperature of about minus 390, or just under 40 degrees above absolute zero.

While the cooling process continues, a 4-minute, 58-second course-correction thruster shot is scheduled for Monday at 2:00 p.m. EST to alter the spacecraft’s speed by a slight 3.4 mph, just enough to place it in a distant orbit around the Lagrange point 2. .

If all goes well, the telescope will stay in this six-month orbit for the rest of its operational life, periodically firing its station-keeping thruster to stay in position.

With the orbital insertion burn behind them, engineers will continue mirror alignment, one of the most complex aspects of Webb’s already complicated deployment.

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

Webb’s main mirror is made up of 18 hexagonal gold-coated beryllium segments that must be aligned to a tiny fraction of the width of a human hair to achieve sharp focus. This photo shows the mirror during pre-launch preparations with the telescope’s secondary mirror folded up for flight. Credit: NASA

After Webb’s near-infrared camera, or NIRCam, cools to its operating temperature, Webb will aim for a bright star so the instrument can map reflections from all 18 segments, creating a mosaic showing their relative size and position.

The mirror segments will then be adjusted one by one, using one actuator and then another, to properly target each one. Additional tiles will be created as the process continues, and depending on the results, the alignment process may need to be repeated.

“The most important thing is to make all 18 primary mirror segments point the same so that their images are roughly the same size,” Feinberg said. “Some of them can be very blurry and so you might get a large spot (blurry star image) 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 out-of-focus images, then move the multiple reflections to the same point at the center of the telescope’s optical axis, all stacked on top of each other to produce a single, highly focused beam of light.

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

The key, he said, is “you really need very good control of these actuators, very precise tilts, because we need these 18 points to overlap very well.”

A given segment can lose one of its six non-impact tilt actuators. Even the loss of a central actuator can be compensated to some extent by slightly moving the segment up or down.

But extensive field testing has shown the high-tech actuators to be extremely reliable. The procedures were tested before launch using a small-scale model of the telescope, and Feinberg said he was confident the alignment process would work as expected.

“When will we have an image of a star in phase (properly stacked and focused)? I think it will be in March, maybe the end of March,” he said.

“But 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 achieving this a full four months into the mission. So , it would be like the end of April.

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

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

This process will take about two more months. Only then will targeted “first light” images be made public.

“We want to make sure that the first images the world sees, humanity sees, do justice to this $10 billion telescope and aren’t of, you know, hey look, a star,” Jane said. Rigby, Project Webb scientist. in Godard.

“So we’re anticipating a series of ‘wow’ images to be released at the end of commissioning when we begin normal science operations designed to show what this telescope can do…and to really knock everyone off their feet.”


Comments are closed.