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NASA shares test image from Webb’s Fine Guidance Sensor, the deepest image of the universe ever captured: Digital Photography Review


We’re growing ever closer to the debut of the first full-color images from the James Webb Space Telescope on July 12. As we near the landmark occasion, NASA has shared a neat preview from Webb’s Fine Guidance Sensor (FGS). The FGS, developed by the Canadian Space Agency, is designed to help the observatory locate and lock onto targets. Still, it turns out it can also capture a mighty fine photo.

FGS’s primary purpose is to enable measurements and imaging by other sensors, but FGS also captures images. However, FGS’s images are often discarded, given the limited communications bandwidth between L2 and Earth. Webb only transmits data from up to two of its scientific instruments simultaneously.

During a week-long stability test in May, bandwidth was available, so the team decided to retrieve images from Webb’s Fine Guidance Sensor. NASA writes, ‘The resulting engineering test image has some rough-around-the-edges qualities to it. It was not optimized to be a science observation; rather, the data was taken to test how well the telescope could stay locked onto a target, but it does hint at the power of the telescope.’ The bright stars in the image have six diffraction spikes due to Webb’s six-sided mirror segments. Beyond the stars are many distant galaxies, some of whom will almost surely be the target of a close investigation by the $10B space telescope in the coming years.

‘This Fine Guidance Sensor test image was acquired in parallel with NIRCam imaging of the star HD147980 over a period of eight days at the beginning of May. This engineering image represents a total of 32 hours of exposure time at several overlapping pointings of the Guider 2 channel. The observations were not optimized for detection of faint objects, but nevertheless the image captures extremely faint objects and is, for now, the deepest image of the infrared sky. The unfiltered wavelength response of the guider, from 0.6 to 5 micrometers, helps provide this extreme sensitivity. The image is mono-chromatic and is displayed in false color with white-yellow-orange-red representing the progression from brightest to dimmest. The bright star (at 9.3 magnitude) on the right hand edge is 2MASS 16235798+2826079. There are only a handful of stars in this image – distinguished by their diffraction spikes. The rest of the objects are thousands of faint galaxies, some in the nearby universe, but many, many more in the distant universe.’

Credit: NASA, CSA and FGS team

The image above is the result of 72 exposures over 32 hours, and it’s among the ‘deepest images of the universe ever taken,’ per Webb scientists. When the FGS’s aperture is open, it doesn’t use color filters, unlike Webb’s other scientific instruments, so that’s why the image is monochromatic. The lack of color filters also means that the image can’t be used for rigorous scientific analysis. However, it’s still a beautiful, impromptu image.

‘With the Webb telescope achieving better-than-expected image quality, early in commissioning we intentionally defocused the guiders by a small amount to help ensure they met their performance requirements,’ said Neil Rowlands, program scientist for Webb’s Fine Guidance Sensor, at Honeywell Aerospace. ‘When this image was taken, I was thrilled to clearly see all the detailed structure in these faint galaxies. Given what we now know is possible with deep broad-band guider images, perhaps such images, taken in parallel with other observations where feasible, could prove scientifically useful in the future.’

NASA adds, ‘The FGS image is colored using the same reddish color scheme that has been applied to Webb’s other engineering images throughout commissioning. In addition, there was no “dithering” during these exposures. Dithering is when the telescope repositions slightly between each exposure. In addition, the centers of bright stars appear black because they saturate Webb’s detectors, and the pointing of the telescope didn’t change over the exposures to capture the center from different pixels within the camera’s detectors. The overlapping frames of the different exposures can also be seen at the image’s edges and corners.’

Webb’s four primary scientific instruments will do a lot of work and help provide incredible new views of the universe. However, for every image Webb captures, the Fine Guidance Sensor will be behind the scenes, helping the space telescope stay on target. The unsung hero of the mission will at least get a few days in the spotlight before July 12.



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