Why do stars look spiky in images from the James Webb Space Telescope

stars in New images from the James Webb Space Telescope It looks sharper than it used to be. And I’m not just talking about the image quality, which is amazing. I’m talking about the fact that many of the bright stars in the photos have very distinct nails that look like a Christmas ornament or, as one of my colleagues put it, “look like a JJ Abrams promotional poster, and I love it.”

But this is not the case Lens flare too much. These are diffraction spikes, and if you look closely, you’ll see that all the bright objects in the JWST images have the same eight-point pattern. The brighter the light, the more obvious the advantage. Faint objects such as nebulae or galaxies Don’t tend to see much of this distortion.

This pattern of yaw spikes is a unique pattern on JWST. If you are Compare photos Taken with the new telescope for images taken by its predecessor, you’ll notice that Hubble has only four diffraction heights to eight at the JWST. (Two of the JWST spikes can be very faint, so sometimes it looks like there are six.)

The shape of the diffraction heights is determined by the telescopes, so let’s start with a quick refresher of the bits of interest. Both Hubble and JWST reflecting telescopes, Which means they collect light from the universe using mirrors. Reflecting telescopes have a large primary mirror that collects light and reflects it back into a smaller secondary mirror. The secondary mirror On space telescopes it helps direct this light towards the scientific instruments that turn it into all the wonderful images and data we see now.

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Both the primary and secondary mirror contribute to diffraction spikes but in slightly different ways. Light is reflected or bent around objects such as the edges of a mirror. So the shape of the mirror itself can produce spikes of light as the light interacts with the edges of the mirror. In Hubble’s case, the mirror was round, so it didn’t add to the fork. But JWST has hexagonal mirrors that result in an image with six diffraction spines.

Photo: NASA

There is also the secondary mirror. Secondary mirrors are smaller than primary mirrors and are held in place away from the primary mirror by means of brackets. In the case of JWST, The struts are 25 feet long. Light passing through these struts is deflected, creating more ripples, each one perpendicular to the strut itself.

In Hubble’s case, its four struts result in the four distinct elevations you see in Hubble images. JWST has three struts holding its secondary mirror, resulting in six more rivets.

JWST struts during cryogenic tests on the ground.
Photo: NASA

This is a lot of distortion. To reduce the number of diffraction spikes, the JWST was designed so that four of the strut-induced protrusions overlapped with four of the mirror-induced protrusions. This leaves the eight diffraction spikes of the soon-to-be iconic JWST image.

Some of the nails will appear more or less visible depending on the tool that also treats the light. This is most evident in the JWST images of the Southern Ring Nebula, released this week.

Two JWST-square views of the Southern Ring Nebula.
Image: NASA, ESA, CSA, and STScI

The image on the left was taken by JWST’s NIRCam, which collects near-infrared light. The image on the right was captured by the telescope’s MIRI instrument, which captures medium-infrared light instead. “In near-infrared light, stars have a more remarkable diffraction spike because they are very bright at these wavelengths,” Explanation Posted by Space Telescope Science Institute says. “In mid-infrared light, diffraction heights also appear around stars, but they are lighter and smaller (zoom in to discover them).”

If you want a visual display of how diffraction heights work on JWST, check out the helpful infographic below From NASA and the Space Telescope Science Institute:

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This graph includes a lot of text. For a text description, Please click here.
Photo: NASA, ESA, CSA, Leah Hostak (STScI), Joseph DePasquale (STScI)


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