This fine-guide sensor test image was acquired in parallel with NIRCam images of the star HD147980 over an eight-day period in early May 2022. This engineering image represents a total of 32 hours of exposure time at various points. Guider 2 channel overlays. The observations were not optimized for detection of faint objects, but the image nonetheless captures extremely faint objects and is, so far, the deepest image of the infrared sky. The guide’s unfiltered wavelength response of 0.6 to 5 micrometers helps provide this extreme sensitivity. The image is monochrome and displayed in false color with white-yellow-orange-red representing the progression from brightest to dimmest. The bright star (magnitude 9.3) on the right 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 team, CSA and FGS.
We are only five days away from July 12the launch of the First full-color images from NASA’s James Webb Space Telescope, but how does the observatory find and lock onto its targets? Webb’s Fine Guidance Sensor (FGS) was designed with this particular question in mind. (The FGS, as well as the Near Infrared Imager and Slitless Spectrograph (NIRISS), were developed by the Canadian Space Agency.) in the coming weeks, months and years.
Although the primary purpose of the FGS is to enable precise scientific measurements and precision pointing imaging, it has always been capable of capturing images. When it does, images are generally not preserved: Given the limited communications bandwidth between L2 and Earth (a distance of 1.5 million kilometers), Webb only sends back data from up to two science instruments at a time. However, during a week-long stability test in May, it occurred to the team that there was data transfer bandwidth available, so they could keep the images being captured.
The James Webb Space Telescope will be the most important space observatory of the next decade, serving astronomers around the world. One of two Canadian elements on the James Webb Space Telescope, the FGS is the most sophisticated guidance sensor of any telescope ever built. It fixates on bright stars in deep space to keep Webb’s images sharp. Credit: CSA
The resulting engineering test image (shown at the top of this article) has some rough qualities. It was not optimized to be a scientific observation; rather, the data was taken to test how well the telescope could stay locked on a target, but it does hint at the power of the telescope. It carries some features of the views that Webb has produced during its post-launch preparations. The bright stars stand out with their six long, sharply defined diffraction peaks, an effect due to Webb’s six-sided mirror segments. Beyond the stars, galaxies occupy almost the entire background.
According to Webb scientists, the result, using 72 exposures over 32 hours, is among the deepest images of the universe ever taken. When the FGS aperture is wide open, it doesn’t use color filters like the other science instruments, which means it’s impossible to study the age of the galaxies in this image with the rigor needed for scientific analysis. But even when capturing unplanned images during a test, FGS is capable of producing stunning views of the cosmos.
the Fine Guide Sensor (FGS) allows Webb to aim precisely, so you can get high-quality images. The FGS/NIRISS Near Infrared Imager and Slitless Spectrograph will be used to investigate the following science objectives: first light detection,[{” attribute=””>exoplanet detection and characterization, and exoplanet transit spectroscopy.
FGS/NIRISS has a wavelength range of 0.8 to 5.0 microns, and is a specialized instrument with three main modes, each of which addresses a separate wavelength range. FGS is a “guider,” which helps point the telescope.
“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. 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,” said Neil Rowlands, program scientist for Webb’s Fine Guidance Sensor, at Honeywell Aerospace.
Since this image was not created with a science result in mind, there are a few features that are quite different than the full-resolution images that will be released on July 12. Those images will include what will be – for a short time at least – the deepest image of the universe ever captured, as NASA Administrator Bill Nelson announced on June 29.
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.
In this engineering test, the purpose was to lock onto one star and to test how well Webb could control its “roll” – literally, Webb’s ability to roll to one side like an aircraft in flight. That test was performed successfully – in addition to producing an image that sparks the imagination of scientists who will be analyzing Webb’s science data, said Jane Rigby, Webb’s operations scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
“The faintest blobs in this image are exactly the types of faint galaxies that Webb will study in its first year of science operations,” Rigby said.
While all four of Webb’s science instruments will ultimately reveal the telescope’s new view of the universe, the fine guidance sensor is the only instrument that will be used on every Webb observation over the course of the mission’s lifetime. FGS has already played a crucial role in Webb’s optics lineup. Now, during the first real science observations in June and once science operations begin in mid-July, he will guide every Webb observation to its target and maintain the precision necessary for Webb to produce groundbreaking discoveries about stars, exoplanets, galaxies, and even moving targets within our solar system.
