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Sunday, August 14, 2022

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

Stars in the new images from the James Webb Space Telescope look sharper than they did before. And I’m not just talking about the image quality, which is astounding. I’m talking about the fact that many of the bright stars in the images have very distinct Christmas-ornament-looking spikes or, as one of my colleagues put it, “It looks like a J.J. Abrams promo poster, and I love it.”

But this isn’t a case of too much lens flare. Those are diffraction spikes, and if you look closely, you’ll see that all bright objects in the JWST images have the same eight-pointed pattern. The brighter the light, the more prominent the feature. Dimmer objects like nebulae or galaxies don’t tend to see quite 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 spikes to JWST’s eight. (Two of JWST’s spikes can be very faint, so it sometimes appears as though there are six.)

The shape of the diffraction spikes is determined by the telescope’s hardware, so let’s start with a quick refresher of the important bits. Both Hubble and JWST are reflecting telescopes, which means that they collect light from the cosmos using mirrors. Reflecting telescopes have a large primary mirror that gathers the light and reflects it back to a smaller secondary mirror. The secondary mirror on space telescopes helps guide that light toward the science instruments that turn it into all the cool images and data we’re seeing now.

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

Image: NASA

There’s also the secondary mirror. Secondary mirrors are smaller than primary mirrors and are held in place some distance away from the primary mirror by struts. In the case of JWST, the struts are 25 feet long. Light passing by these struts gets diffracted, resulting in more spikes, each one perpendicular to the strut itself.

In Hubble’s case, its four struts resulted in the four distinct spikes you see in Hubble pictures. JWST has three struts holding up its secondary mirror, resulting in another six spikes.

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

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

Some of the spikes will look more or less visible depending on which instrument is processing the light as well. This is most noticeable in the JWST images of the Southern Ring Nebula, which were released this week.

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

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

If you want a visual of how diffraction spikes on JWST work, check out the handy infographic below from NASA and the Space Telescope Science Institute:

This is a diagram labeled “Webb’s Diffraction Spikes”. The top right of the image shows three stars producing eight-pronged diffraction spike patterns. This diagram is composed of five sections.

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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