Why stars look spiky in James Webb Space Telescope images

Why stars look spiky in James Webb Space Telescope images
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stars in the New images from the James Webb Space Telescope they look sharper than before. And I’m not just referring to the image quality, which is amazing. I am referring to the fact that many of the bright stars in the images have very distinctive spikes that look like Christmas decorations or, as one of my colleagues put it, “It looks like a promotional poster for JJ Abrams, and I love it. ”

But this is not a case of too much lens flare. Those 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 prominent the feature. Faint objects like nebulae or galaxies you don’t tend to see as much of this distortion.

This pattern of diffraction spikes is unique to JWST. If you compare images taken by the new telescope to images taken by its predecessor, you’ll notice that Hubble only has four diffraction peaks versus JWST’s eight. (Two of the JWST peaks can be very faint, so it sometimes appears that there are six.)

The shape of the diffraction peaks is determined by the hardware of the telescope, so let’s start with a quick review of the important parts. Both Hubble and JWST are reflecting telescopes, which means that they collect the light of the cosmos through mirrors. Reflecting telescopes have a large primary mirror that collects light and reflects it back to a smaller secondary mirror. the secondary mirror in space telescopes it helps guide that light into the science instruments that turn it into all the great images and data we’re seeing right now.

Both the primary and secondary mirrors contribute to the diffraction peaks, but in slightly different ways. Light diffracts, or bends, around objects like the edges of a mirror. So the shape of the mirror itself can result in these spikes of light when the light interacts with the edges of the mirror. In Hubble’s case, the mirror was round, so it didn’t add up to the pointy one. But JWST has hexagonal mirrors that result in an image with six diffraction peaks.

an illustration of a person standing next to the round silver hubble mirror.  on the left is the much larger JWST primary mirror

Image: NASA

There is also the secondary mirror. Secondary mirrors are smaller than primary mirrors and are held in place at a distance from the primary mirror by struts. In the case of JWST, struts are 25 feet long. Light passing through these struts is diffracted, resulting in more peaks, each perpendicular to the strut itself.

In Hubble’s case, its four struts resulted in the four distinct peaks seen in the Hubble images. JWST has three struts supporting its secondary mirror, resulting in another six spikes.

JWST with their struts during cryogenic testing on Earth.
Image: NASA

That’s a lot of distortion. To minimize the number of diffraction peaks, JWST was designed so that four of the peaks caused by the struts overlapped with four of the peaks caused by the mirror. That leaves the eight soon-to-be-iconic diffraction spikes of a JWST image.

Some of the peaks will be more or less visible depending on which instrument is also processing the light. This is most noticeable in the JWST images of the South Ring Nebula, which were released this week.

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

The image on the left was taken by JWST’s NIRCam, which captures near-infrared light. The one on the right was taken by the telescope’s MIRI instrument, which instead captures mid-infrared light. “In near-infrared light, stars have more prominent diffraction peaks because they are so bright at these wavelengths,” a Explanation published by the Space Telescope Science Institute says. “In mid-infrared light, diffraction peaks also appear around stars, but they are fainter and smaller (zoom in to spot them).”

For a picture of how diffraction spikes work in JWST, check out the handy infographic below. NASA and the Space Telescope Science Institute:

This is a diagram called

This infographic includes a lot of text. For a text-based description, please click here.
Image: NASA, ESA, CSA, Leah Hustak (STScI), Joseph DePasquale (STScI)


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