Why do stars appear dark in images from the James Webb Space Telescope?

Why do stars appear dark in images from the James Webb Space Telescope?
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Stars New images from the James Webb Space Telescope they look sharper than before. I’m not just talking about the picture quality, which is amazing. I’m talking about the fact that most of the bright stars in the pictures have spikes that look like very different Christmas ornaments, or as one of my colleagues said, “It looks like a JJ Abrams promo poster and I love it. ”

But this is not so 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 octagonal pattern. The brighter the light, the more noticeable the feature. Reduce objects such as nebulae or galaxies don’t tend to see too much of this distortion.

This pattern of diffraction spikes is typical of JWST. If you compare the pictures superimposed by the new telescope on images taken by its predecessor, you’ll see that Hubble has just four diffraction spikes to JWST’s eight. (Two of JWST’s spikes can be very faint, so it sometimes looks like there are six.)

The shape of the diffraction spikes is determined by the hardware of the telescope, so let’s start with a quick refresher on the important bits. Both Hubble and JWST reflecting telescopes, this means they collect light from space using mirrors. Reflecting telescopes have a large primary mirror that collects light and reflects it onto a smaller secondary mirror. The secondary mirror in space telescopes, it helps direct that light into the scientific instruments that turn it into all the beautiful images and data we see now.

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

image of a man standing next to the round, silver hubble mirror.  on the left is the larger JWST primary mirror

Photo: NASA

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

In Hubble’s case, its four supports resulted in the four distinct spikes you see in Hubble images. JWST has three spikes holding its secondary mirror, resulting in six more spikes.

JWST with props during cryogenic testing on Earth.
Photo: NASA

This is very misleading. To minimize the number of diffraction spikes, the JWST is designed so that four of the spikes generated by the supports coincide with four spikes generated by the mirror. This leaves the eight diffraction spikes that will soon become iconic in the JWST image.

Depending on which instrument is processing the light, some spikes will be more or less visible. This is most noticeable 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 instrument, which collects near-infrared light. The one on the right was taken by the MIRI instrument, which captures mid-infrared light instead of a telescope. “In near-infrared light, stars have more prominent diffraction spikes because they are so bright at these wavelengths” explanation This was reported by the Space Telescope Science Institute. “Diffraction spikes in mid-infrared light are also visible around stars, but they are fainter and smaller (zoom in to detect them).”

If you want to see how diffraction works at JWST, check out the helpful infographic below From NASA and the Space Telescope Science Institute:

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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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