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

Why do stars appear pointed 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. And I’m not just talking about the image quality, which is amazing. I’m talking about the fact that most of the bright stars in the images have very prominent spikes like Christmas ornaments, or as one of my colleagues put it, “It looks like a JJ Abrams promotional poster and I love it. ”

But this is not the case too much lens flare. These are diffraction increments, and if you look closely, you’ll see that all the bright objects in the JWST images have the same eight-dot pattern. The brighter the light, the more pronounced the feature. fainter objects such as nebulae or galaxies Don’t be inclined to see much of this distortion.

This diffraction spike pattern is unique to JWST. if you compare pictures Taken by the new telescope into images taken by its predecessor, you’ll notice that Hubble has only four diffraction spikes versus eight of the JWST. (Two of the 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 telescope’s hardware, so let’s start with a quick refresher of the important bits. Both Hubble and JWST reflecting telescopes, which means they collect light from the universe using mirrors. Reflecting telescopes have a large primary mirror that collects light and reflects it to a smaller secondary mirror. this secondary mirror Space telescopes help direct that light into scientific instruments that transform it into all the wonderful images and data we’re seeing now.

Both primary and secondary mirrors contribute to their diffraction boosts, but in slightly different ways. Light is refracted or bent around objects such as mirror edges. Therefore, as the light interacts with the edges of the mirror, the shape of the mirror can cause these light bounces. In Hubble’s case, the mirror was round, so it didn’t add to the sharpness. However, JWST has hexagonal mirrors that result in an image with six diffraction spikes.

A picture of a person standing next to a round, silvery-bellied mirror.  on the left is the much larger JWST primary mirror

Image: NASA

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

In Hubble’s case, its four supports resulted in four different spikes that you see in the Hubble pictures. The JWST has three struts holding its secondary mirror, resulting in six spikes.

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

That’s too much distortion. To minimize the number of diffraction spikes, the JWST was designed so that four of the spikes caused by the perpendiculars overlap with four of the spikes caused by the mirror. This leaves the eight diffraction spikes that will soon become iconic of a JWST image.

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

Two JWST images 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 picked up mid-infrared light. “In near-infrared light, stars have more pronounced diffraction spikes because they are so bright at these wavelengths.” explanation Says published by the Space Telescope Science Institute. “In mid-infrared light, diffraction spikes are also visible around stars, but they are fainter and smaller (zoom in to detect them).

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

This is a diagram labeled

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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