This is why the first image of JWST looks full of "lens flare"

The stars in new images from the James Webb Space Telescope look sharper than ever before. And I’m not just talking about the image quality, which is amazing. I talk about the fact that many of the bright stars in the picture have very different nails from the Christmas decorations or, as one of my colleagues put it, “This looks like a JJ Abrams promo poster, 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 image have the same eight-pointed pattern. The brighter the light, the more prominent the features. Dimmer objects such as nebulae or galaxies are less likely to see this distortion.

This diffraction spike pattern is unique to JWST. If you compare the images taken by the new telescope with the images taken by its predecessors, you will see that Hubble only has four diffraction spikes for the eight JWSTs. (Two JWST spikes can be very dim, so sometimes it appears as if there are six.)

From now on you will always be able to tell the difference between a Hubble image and a JWST image:

The Hubble star has four spikes in the cross. The JWST star has six in the snowflake. Thank you for your time. pic.twitter.com/BWsv2WqCqD

— Hank Green (@hankgreen) 12 July 2022

The shape of the diffraction spikes is determined by the telescope’s hardware, so let’s start with a quick refresh of the important parts. Both Hubble and JWST are reflecting telescopes, which means they collect light from the cosmos using mirrors. Reflecting telescopes have a large primary mirror that collects light and reflects it back onto a smaller secondary mirror. The secondary mirrors on the space telescope help direct that light to science instruments that turn it into all the cool pictures and data we see today.

Both primary and secondary mirrors contribute to diffraction spikes but in slightly different ways. Light diffracts, or bends, around an object such as the edge of a mirror. So the shape of the mirror itself can produce these spikes of light as the light interacts with the edges of the mirror. In Hubble’s case, the mirror is round, so it doesn’t add to the tape. But JWST has a hexagonal mirror that produces an image with six diffraction spikes.

Image: NASA

There is also a secondary mirror. The secondary mirror is smaller than the primary mirror and is held in place some distance from the primary mirror by a support. In the case of JWST, the struts are 25 feet long. Light passing through these struts is diffracted, producing more spikes, each perpendicular to the strut itself.

In the case of Hubble, the four supports make up the four different spikes you see in the Hubble image. The JWST has three supports that hold its secondary mirror in place, producing another six nails.

JWST with its supports during cryogenic testing on Earth.

Image: NASA

That’s a lot of distortion. To minimize the number of diffraction nails, JWST was engineered so that the four nails caused by the struts would overlap the four nails caused by the mirror. That left eight diffraction spikes that would soon become the icon of the 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 visible in the 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 is picked up by the telescope’s MIRI instrument, which picks up center-infrared light instead. “In near-infrared light, stars have more pronounced diffraction spikes because they are very bright at this wavelength,” an explanation posted by the Space Telescope Science Institute said. “In mid-infrared light, diffraction spikes also appear around the star, but they are fainter and smaller (zoom in to see).”

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

This infographic includes a lot of text. For text-based descriptions, please click here.

Image: NASA, ESA, CSA, Leah Hustak (STScI), Joseph DePasquale (STScI)

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