Webb reaches Lagrange Point 2, setting stage for mirror alignment

An artist’s impression of the James Webb Space Telescope in operation. Image: NASA

On 24 January, 30 days after launch on Christmas Day, the James Webb Space Telescope slipped into orbit around Lagrange Point 2, firing a small manoeuvring thruster for about 5 minutes to complete a 1.5-million-kilometre (907,500-mile) voyage from Earth.

Now in a six-month orbit around Lagrange Point 2, or L2, Webb will circle the sun in gravitational lockstep with Earth, able to remain on station with a minimum amount of rocket fuel.

The orbit also will allow Webb to observe the universe while keeping its tennis court-size sunshade broadside to the Sun, blocking out heat and light from the inner solar system that otherwise would overwhelm the telescope’s sensitive instruments.

This NASA representation shows Webb’s location at the time of a thruster firing – MCC-2 – to ease into orbit around Lagrange Point 2. Image: NASA

With Webb now safely on station, the mission operations team at the Space Telescope Science Institute in Baltimore, Maryland, will focus on precisely aligning the 18 segments making up its 6.5-metre (21.3-foot) primary mirror.

To achieve the razor-sharp focus required for its astronomical observations, the mirror segments must be aligned to within a tiny fraction of the width of a human hair using high-tech actuators – seven per segment – to make tiny changes in tilt and to adjust the curvature if needed.

Even though the sunshade is cooling the telescope as planned and temperatures on the backside of the shield are already below minus 200 Celsius (about minus 330 F), “all 132 actuators are working completely nominally,” said Lee Feinberg, Webb optical element manager at NASA’s Goddard Space Flight Center.

The alignment process will begin after one of Webb’s instruments, the Near Infrared Camera, or NIRCam, cools down enough for its sensors to operate normally. At that point, Webb will be aimed at HD 84406, a 6.9-magnitude star in Ursa Major, and NIRCam will be used to map the reflections from all 18 mirror segments.

“We’ll get 18 separate images that will be very blurry because these individual (segments) won’t be aligned,” Feinberg said. “So we’ll align them, and then we will put those 18 images on top of each other. We call that stacking.”

The alignment will be fine-tuned so the 18 segments act as a single mirror, working with the 0.7-metre (2.4-foot) secondary mirror to send a single precisely focused beam to Webb’s four instruments.

The alignment process, adjusting the position of one segment at a time, is expected to take about three months to complete. After that, the team plans to spend another two months calibrating NIRCam, three spectrographs and Webb’s fine-guidance sensor.

The first science images are expected in the June timeframe.

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

“So we are planning a series of ‘wow’ images to be released at the end of commissioning when we start normal science operations that are designed to showcase what this telescope can do … and to really knock everybody’s socks off.”