Stars in new images from the James Webb Space Telescope appear sharper than 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 spikes that look like Christmas ornaments, or, as one of my colleagues said, “It looks like a JJ Abrams poster and I love it. ”
But this is not a case of too much lens flare. 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-beam pattern. The brighter the light, the more noticeable the feature. Faint objects like nebulae or galaxies don’t tend to see as much of this distortion.
This pattern of diffraction peaks is unique to JWST. If you compare images taken by the new telescope with images taken by its predecessor, you’ll notice that Hubble has only four diffraction peaks compared to JWST’s eight. (Two of JWST’s spikes can be very faint, so it sometimes looks like there are six.)
From this point on, you will always be able to tell the difference between a Hubble image and a JWST image:
Hubble’s stars have four spikes in a cross. JWST stars have six in one snowflake. Thank you for your time. pic.twitter.com/BWsv2WqCqD
— Hank Green (@hankgreen) July 12, 2022
The shape of the diffraction spikes is determined by the telescope hardware, so let’s start with a quick refresher on the important parts. Both Hubble and JWST are reflecting telescopes, meaning they collect light from space using mirrors. Reflector telescopes have a large primary mirror that collects the light and reflects it back to a smaller secondary mirror. The secondary mirror on space telescopes helps direct that light to the scientific instruments that turn it into all the great images and data we see now.
Both the primary and secondary mirrors contribute to the diffraction peaks, but in slightly different ways. Light is diffracted or bent around objects such as mirror edges. So the very shape of the mirror can cause these light peaks 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 sharpness. But JWST has hexagonal mirrors that result in an image with six diffraction peaks.
Image: NASA
There is also a secondary mirror. Secondary mirrors are smaller than primary mirrors and are held in place some distance from the primary mirror by struts. In the case of JWST, the struts are 25 feet long. Light passing through these struts is diffracted, resulting in more spikes, each perpendicular to the strut itself.
In Hubble’s case, its four struts gave rise to the four distinct spikes you see in the Hubble pictures. JWST has three struts holding the secondary mirror, leading to six more studs.
JWST with its struts during cryogenic tests on Earth. Image: NASA
That’s a lot of distortion. To minimize the number of diffraction peaks, the JWST is 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 will appear more or less visible depending on which tool is also processing the light. This is most noticeable in the JWST images of the Southern Ring Nebula that 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 collects near-infrared light. The one on the right was taken by the telescope’s MIRI instrument, which instead captures mid-infrared light. “In the near-infrared, stars have more prominent diffraction peaks because they are so bright at these wavelengths,” says an explanation published by the Space Telescope Science Institute. “In the mid-infrared, diffraction spikes also appear around the stars, but they are fainter and smaller (zoom in to see them).”
If you want a visual of how JWST’s diffraction peaks work, check out the handy infographic below from NASA and the Space Telescope Science Institute:
This infographic includes a lot of text. For a text description, please click here. Image: NASA, ESA, CSA, Leah Hustak (STScI), Joseph DePasquale (STScI)
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