James Webb’s Guidance Sensor Provides Preview of its Imaging Prowess

NASA has released a new image that was captured by the James Webb Space Telescope’s Fine Guidance Sensor (FGS), the tool the observatory finds and locks onto targets, which provides insight into the incredible imaging feats of the telescope.

In less than a week, NASA will release the first color images taken by the James Webb Space Telescope, but ahead of that momentous announcement, the space organization provided details on how the observatory acquires targets captured by its main imagery. system, and the image quality of this navigation sensor is another indication of Webb’s incredible imaging ability.

NASA explains that the FGS has always been able to capture photos, but its main task is to enable precise scientific measurements and imaging with precise pointing. When he takes pictures, those images are usually not retained given the limited communication bandwidth between him and Earth. NASA says Webb only sends data from two science instruments at a time, but during a test in May the team realized they could keep the images that were sent because there was a band additional bandwidth available.

And they did.

Webb FGS Sensor
This fine guidance sensor test image was acquired in parallel with NIRCam imagery of the star HD147980 over an eight-day period in early May. This engineering image represents a total of 32 hours of exposure time to multiple overlapping pointings of the Guider 2 channel. Observations were not optimized for faint object detection, but nonetheless the image captures objects extremely weak and is, for now, 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 monochromatic and is displayed in false color with white-yellow-orange-red representing the progression from lightest to darkest. The bright star (of magnitude 9.3) on the right edge is 2MASS 16235798+2826079. There are only a handful of stars in this image, which are distinguished by their diffraction peaks. The rest of the objects are thousands of faint galaxies, some in the near universe, but many, many more in the far universe. | Credit: NASA, ASC and FGS team.

“The resulting engineering test image exhibits rough qualities. It was not optimized to be a scientific observation; instead, the data was taken to test how well the telescope could remain locked on a target, but it alludes to the power of the telescope,” NASA writes.

“It carries some characteristics of the sights Webb produced during his post-launch preparations. Bright stars are distinguished by their six long, well-defined diffraction spikes – an effect due to Webb’s six-sided mirror segments. Beyond the stars, galaxies fill almost the entire background.

The purpose of the test that these images are from was to lock onto a star and show just how “rolling” Webb could be – NASA literally means that, as Webb will roll to one side like an airplane in flight. NASA reports that the test was successful.

The photo above consists of 72 exposures that were captured over the course of 32 hours and, despite being taken by the FGS, is one of the deepest images of the universe ever taken. On July 12, the first color images revealed by the telescope will be the most distant ever captured.

“When the aperture of the FGS is open, it does not use color filters like other scientific instruments, which means that it is impossible to study the age of the galaxies in this image with the rigor necessary for the study. scientific analysis. But even when capturing unplanned images during a test, FGS is able to produce stunning views of the cosmos,” adds NASA.

NASA explains that since the photo was not created with science in mind, there are notable differences from the full-resolution images that will be released next week. Although the image has the same reddish color seen in previous Webb images, there is no “dithering” – when the telescope repositions itself slightly between each exposure – in these exposures. The centers of the stars in the photo appear black because they have saturated Webb’s sensors, and overlapping frames from the different exposures can be seen at the edges and corners of the image.

“With the Webb telescope achieving better than expected image quality, at the start of commissioning we intentionally defocused the guides by a small amount to ensure they met their performance requirements. When this image was taken, I was delighted to clearly see all the detailed structure of these faint galaxies,” says Neil Rowlands, Webb’s Fine Guidance Sensor Program Scientist, Honeywell Aerospace.

“Given what we now know is possible with deep broadband guidance images, perhaps such images, taken in parallel with other observations when possible, could prove scientifically useful to the coming.”

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