Canadian built sensor successfully performs first James Webb Space Telescope alignment

The Canadian Space Agency (CSA) is reporting the successful use of the Canadian built Fine Guidance Sensor (FGS) on the James Webb Space Telescope used to align the telescope’s mirrors.

The CSA said today that “On February 13, the Webb team performed “Line of Sight” testing that confirmed the FGS’s ability to “lock on” to a specific guide star in tracking mode. This mode allows the FGS to transmit highly precise information to Webb’s positional system 16 times per second.”

René Doyon, the principal investigator for FGS/NIRISS (Near-Infrared Imager and Slitless Spectrograph), and Nathalie Ouellette, Webb outreach scientist, of the Université de Montréal provided NASA with an explanation on how Webb uses the FGS in this process.

“After being powered on Jan. 28, 2022, and undergoing successful aliveness and functional tests, Webb’s Fine Guidance Sensor (FGS) has now successfully performed its very first guiding operation! Together with the Near-Infrared Imager and Slitless Spectrograph (NIRISS), the FGS is one of Canada’s contributions to the mission.”

“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. In addition to its speed, the FGS also needs to be incredibly precise. The degree of precision with which it can detect changes in the pointing to a celestial object is the equivalent of a person in New York City being able to see the eye motion of someone blinking at the Canadian border 500 kilometers (311 miles) away!”

This image mosaic was created by pointing the telescope at a bright, isolated star in the constellation Ursa Major known as HD 84406. This star was chosen specifically because it is easily identifiable and not crowded by other stars of similar brightness, which helps to reduce background confusion. Each dot within the mosaic is labeled by the corresponding primary mirror segment that captured it. These initial results closely match expectations and simulations. Credit: NASA.

“Webb’s 18 primary mirror segments are not yet aligned, so each star appears as 18 duplicate images. On Feb. 13, FGS successfully locked onto and tracked one of these star images for the first time. The FGS team was thrilled to see this ‘closed loop guiding’ working! From now on, most of the alignment process of the telescope mirrors will take place with FGS guiding, while NIRCam images provide the diagnostic information for mirror adjustments.”

The CSA also stated today that “in the coming weeks, with the help of the FGS, each mirror segment will be carefully adjusted to ‘stack’ these views and calibrate the rest of the telescope’s optical elements, to ultimately create a highly focused image of a single star.”