Why stars look pointed in James Webb Space Telescope images

Stars in the new James Webb Space Telescope images look sharper than before. And I’m not just talking about the picture quality, which is amazing. I’m talking about the fact that many of the bright stars in the images have very distinct spikes that look like Christmas ornaments, or as one of my colleagues put it, “It looks like a JJ Abrams commercial and I love it. ”

But that’s not a case of too much lens flare. These are diffraction peaks, and if you look closely you’ll see that all of the bright objects in the JWST images share the same eight-point pattern. The brighter the light, the more prominent the feature. Darker objects like nebulae or galaxies usually don’t see as much of this distortion.

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

The shape of the diffraction peaks is determined by the telescope’s hardware, so let’s start with a quick refresher on the important parts. Both Hubble and JWST are reflecting telescopes, meaning they use mirrors to collect light from the cosmos. Reflecting telescopes have a large primary mirror that collects the light and bounces it back to a smaller secondary mirror. Space telescopes’ secondary mirrors help funnel that light to the scientific instruments, which convert it into all the cool images and data we see now.

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

an image 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 by struts some distance from the primary mirror. In the case of JWST, the struts are 25 feet long. Light passing these struts is diffracted, resulting in more spikes, each perpendicular to the strut itself.

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

JWST with its struts during cryotesting 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 overlap with four of the peaks caused by the mirror. This leaves the eight soon-to-be-iconic diffraction peaks of a JWST image.

Some of the spikes look more or less visible depending on which instrument is processing the light. This is most evident in JWST images of the Southern Ring Nebula 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 collects near-infrared light. The one on the right was captured by the telescope’s MIRI instrument, which picks up mid-infrared light instead. “In the near-infrared, stars have more noticeable diffraction peaks because they are so bright at those wavelengths,” the Space Telescope Science Institute said in a statement. “In the mid-infrared, diffraction peaks also appear around stars, but they are fainter and smaller (zoom in to see them).”

If you want to get an idea of ​​how diffraction peaks work at JWST, check out the handy infographic below from NASA and the Space Telescope Science Institute:

This is a diagram labeled Webb's Diffraction Peaks.  Three stars can be seen in the upper right of the image, creating eight-point diffraction peak patterns.  This chart consists of five sections.

This infographic contains 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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