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JWST with IR shield deployed

How to Tell the Difference Between Hubble and Webb Images

The Difference Between Hubble and Webb Images - Illustration of the JWST in space
Artist’s rendition of the JWST in space

More than Meets the Eye

The difference between Hubble and Webb images is more than simple clarity or resolution. Here is a simple way to tell the difference between images taken by the older. The most glaring difference—pun intended—are the eight-pointed diffraction spikes jutting out from some stars in the image.

Telling the Difference Between Hubble &Webb Images

Eight-Pointed Diffraction Spikes

Diffraction spikes—the flares pointing out from some of the brightest stars—are interference patterns that come from light scattered off mirror struts that support the secondary mirror on the telescope. The edges of all the eighteen hexagonal mirrors on the Webb create additional diffraction patterns. Since these flares result from interference patterns, the intensity of the spikes depends on the brightness and wavelength of the object observed.

WHAT ARE DIFFRACTION SPIKES? Have you ever noticed that bright stars in your favorite space images have unique spikes around them? These are known as diffraction spikes. Diffraction spikes are patterns produced as light bends around the sharp edges of a telescope. While all stars can create these patterns, we only see spikes with the brightest stars when a telescope takes an image. For most reflecting telescopes, including Webb, diffraction spikes appear when light interacts with the primary mirror and struts that support the secondary mirror.
HOW DOES DIFFRACTION HAPPEN? Light, which has wave-like properties, tends to radiate from a central point outward, similar to how water behaves when you toss a stone into it. A light encounters an edge. It is bent and redirected, sending it in different directions. In situations where these light waves meet and interact, they can either become more amplified or cancel each other each other out. These areas of amplification and cancellation from the light and dark spots that show in diffraction patterns.
PRIMARY MIRROR INFLUENCE Primary mirrors in reflecting telescopes cause light waves to interact as they direct light to the secondary mirror. So, even if a telescope had no struts, it would still create a diffraction pattern. The shape of the primary mirror, in particular the number of edges it has, determines the mirror's diffraction pattern. Light waves interact with those edges to create perpendicular diffraction spikes.
STRUT INFLUENCE The number and position of struts holding up the secondary mirror determine the struts' diffraction spike pattern. In the first row, there is a set of struts organized in a single line. When light hits a strut, the light bends into a single, perpendicular pattern of amplified and canceled light (represented by a yellow dashed line). In the second row, it shows a second set of struts added to the first, creating a second, perpendicular diffraction spike (represented by a red dashed line). In the third row, there are three struts with two of them at an angle. In this case, there would be three diffraction spikes, with each spike perpendicular to a strut (represented by yellow, red, and blue dashed lines).
WEBB'S EIGHT-POINTED STARS Like most reflecting telescopes, the Webb’s primary mirror and struts define its diffraction spikes. Webb has three struts, with two angled at 150 degrees from its vertical strut, and each of the hexagonal segments in its primary mirror contain edges for light to diffract against. Webb's struts are designed so that their diffraction spikes partially overlap with those created by the mirrors. Both of these lead to Webb's complex eight-pointed star pattern.
Source: Space Telescope Science Institute

One More Difference: The Snowflake Pattern

The image below does not show a new type of star or an alien spacecraft. The snowflake-like arrangement in the star appears to mimic the mirror’s structure in various colors. Different wavelengths show as separate variations of the interference patterns.

Snowflake pattern

More about Space Telescopes

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