We’re less than a week away from releasing the first full-color images from NASA’s James Webb Space Telescope, but how does the observatory find and lock on to its targets? Webb’s Fine Guidance Sensor (FGS) – developed by the Canadian Space Agency – was designed with this particular question in mind. It recently captured a view of stars and galaxies that gives a tantalizing glimpse of what the telescope’s science instruments will reveal in the coming weeks, months and years.
The FGS has always been capable of capturing images, but its primary purpose is to enable precise scientific measurements and precisely targeted imaging. When it captures images, the images are usually not saved: given the limited bandwidth of communication between L2 and Earth, Webb only sends data from up to two science instruments at a time. But during a week-long stability test in May, it occurred to the team that they could keep the captured images because there was bandwidth available to transfer data.
The resulting image from the engineering test has some rough qualities. It is not optimized to be a scientific observation; rather, the data was taken to test how well the telescope could stay locked onto the target, but it does hint at the power of the telescope. It bears several hallmarks of the views that Webb produced during his post-release build-up. The bright stars stand out with their six long, well-defined diffraction peaks, an effect due to Webb’s six-sided mirror segments. Beyond the stars, galaxies fill almost the entire background.
The result — using 72 exposures over 32 hours — is among the deepest images of the universe ever taken, according to Webb scientists. When the FGS’s aperture is open, it doesn’t use color filters like other science instruments — meaning it’s impossible to probe the ages of the galaxies in this image with the rigor needed for scientific analysis. But even when it captures unplanned images during a test, the FGS is capable of creating stunning views of space.
“With the Webb telescope achieving better than expected image quality, early in the commissioning we deliberately defocused the guides by a small amount to ensure they met their performance requirements. When this image was taken, I was excited to see clearly all the detailed structure in these faint galaxies. “Given what we now know is possible with deep broadband guidance images, perhaps such images, taken in parallel with other observations where possible, could prove scientifically useful in the future,” Neal said Rowlands, program scientist for Webb’s Fine Guidance Sensor, at Honeywell Aerospace.
Because this image was not created with a scientific output, there are a few features that are quite different from the full-resolution images that will be released on July 12. Those images will include what will be — at least for a short time — the deepest image of the universe ever taken, NASA Administrator Bill Nelson announced on June 29.
The FGS image is colored using the same reddish color scheme that was applied to the other Webb engineering images during commissioning. Also, there was no “flickering” during these exposures. Dithering is when the telescope is repositioned slightly between each exposure. In addition, the centers of bright stars appear black because they saturate Webb’s detectors, and the telescope’s pointing does not change across exposures to capture the center from different pixels in the camera’s detectors. Overlapping frames of different exposures can also be seen at the edges and corners of the image.
In this engineering test, the goal was to fixate on a star and test how well Webb could control its “rollover”—literally, Webb’s ability to roll to one side like an airplane in flight. That test was successful — in addition to creating an image that ignites the imaginations of scientists who will analyze Webb’s science data, said Jane Rigby, Webb’s operations scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
“The faintest patches in this image are exactly the types of faint galaxies that Webb will study in its first year of science operations,” Rigby said.
While Webb’s four science instruments will eventually reveal the telescope’s new view of the universe, the fine-tuning sensor is the only instrument that will be used in every single Webb observation over the life of the mission. The FGS has already played a crucial role in aligning Webb’s optics. Now, during the first real science observations made in June and after science operations begin in mid-July, it will direct every Webb observation to its target and maintain the precision Webb needs to make breakthrough discoveries of stars, exoplanets , galaxies and even moving targets in our solar system.
By Patrick Lynch, NASA Goddard Space Flight Center, Greenbelt, Maryland.