James Webb Space Telescope has locked onto guide star in crucial milestone

By Elizabeth Howell

The James Webb Space Telescope’s key pointing instrument is working well in testing, according to two space agencies involved with the observatory’s commissioning work. The Fine Guidance Sensor (FGS), a contribution from Honeywell on behalf of the Canadian Space Agency, successfully “locked on” to a specific guide star in tracking mode on Sunday (Feb. 13), the CSA reported Thursday (Feb. 17).

With FGS working well so far, the instrument will next be used to assist with the ongoing alignment of the 18 hexagonal segments that make up the primary mirror of the telescope, the CSA added. Last week, Webb engineers released a first image of a single star showing many separate views from the different mirror segments, which was expected since alignment is ongoing.

“In the coming weeks, with the help of the FGS, each mirror segment will be carefully adjusted to ‘stack’ these [individual segment] views and calibrate the rest of the telescope’s optical elements, to ultimately create a highly focused image of a single star,” the CSA said.

A NASA statement about the milestone posted on Thursday included commentary from both René Doyon (principal investigator of the Canadian-built instruments on Webb) and outreach scientist Nathalie Ouellette of the Université de Montréal. The two scientists discussed the function of FGS and what to expect in the coming weeks.

“To ensure Webb stays locked on its celestial targets, the FGS measures the exact position of a guide star in its field of view 16 times per second and sends adjustments to the telescope’s fine steering mirror about three times per second,” the scientists said in the statement.

Speaking about the milestone, the scientists said they were “thrilled” that the guiding is working well so far; they are also looking forward to FGS continuing to assist Webb’s Near-Infrared Camera (NIRCam) with pointing in the right direction.

“From now on, the alignment process of the telescope mirrors will take place with FGS guiding, while NIRCam images provide the diagnostic information for mirror adjustments,” the scientists added.

Webb’s commissioning period is expected to take several months, in large part because the instruments are still cooling after its Dec. 25 launch and month-long journey to a Lagrange point deep in space. The instruments are designed to operate at minus 384 degrees Fahrenheit (minus 233 degrees Celsius) as Webb seeks out heat-emitting objects in the universe.

Elizabeth Howell, Ph.D., is a contributing writer for Space.com since 2012. As a proud Trekkie and Canadian, she tackles topics like spaceflight, diversity, science fiction, astronomy and gaming to help others explore the universe. Elizabeth’s on-site reporting includes two human spaceflight launches from Kazakhstan, and embedded reporting from a simulated Mars mission in Utah. She holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, and a Bachelor of Journalism from Canada’s Carleton University. Her latest book, NASA Leadership Moments, is co-written with astronaut Dave Williams. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Follow Elizabeth Howell on Twitter @howellspace. Follow us on Twitter @Spacedotcom and on Facebook.

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