 The most frequently asked question that I get about #JWST images is: "Why do the stars look like that?". In other words: why are the stars "spikey"? The answer has to do with both the wave nature of light, the shape of the telescope's primary mirror, and the position of the struts that hold up the secondary mirror. The full infographic on JWST diffraction spikes can be downloaded here: https://webbtelescope.org/contents/media/images/01G529MX46J7AFK61GAMSHKSSN (1/7) 5 comments
This is because light, among other things, is a wave. Light waves radiate from a point outward, similar to how water waves ripple when a stone is tossed into a pond. As light encounters an edge, it is bent and redirected. In situations where these light waves meet and interact, they can interfere constructively and destructively, amplifying or canceling each other out. This makes light and dark spots that show in diffraction patterns. (3/7) Primary mirrors in reflecting telescopes cause light waves to interact as they direct light to the secondary mirror. Even if a telescope had no struts, it would still create a diffraction pattern. The shape of the mirror, and any edges it has, determine its pattern. In general, you will get one spike per edge of your mirror. So, JWST's 6-sided mirror makes a 6-pointed pattern. (4/7) The number and position of struts holding up the secondary mirror determine the struts’ diffraction spike pattern. When light hits a strut, the light bends into a single, perpendicular pattern shown here by yellow/red/blue dashed lines. Hubble's four struts make a cross-shaped pattern. JWST's three struts make a squished, 6-pointed star pattern. (5/7) JWST’s eight-pointed diffraction spikes are made by two overlapping, 6-pointed patterns, a stronger one from the mirror and a fainter one from the struts. (6/7) |
Gregory's Server
The unique spikes around the bright stars in your favorite space images are known as diffraction spikes. For most reflecting telescopes, including JWST, diffraction spikes appear when light interacts with the primary mirror and struts that support the secondary mirror. While all stars can create these patterns, we only see spikes with the brightest stars (or point-like objects) when a telescope takes an image. (2/7)