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Kelly Lepo

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: webbtelescope.org/contents/med

(1/7)

#thread #astrodon #astronomy

At the bottom left of this vertical image are the thickest regions of brown and rusty red gas and dust. There are many layers of semi-transparent gas and dust overlaying one another. A peak rises about a third of the way from the bottom, and becomes far darker brown with two bright red areas toward the tip. The light brown dust becomes more diaphanous about halfway up the screen. There’s a slight gap in the dust, which allows the blue background to come into view clearly. About 60% of the background in this image is set in shades of blue and littered with tiny yellow and blue stars. The brown pillars continue, taking the shape of a shoulder at the base, with three prominent columns rising out toward the upper right. The top left pillar is the largest and widest. The peaks of the second and third pillars are set off in darker shades of brown and have red outlines.
8 Nov 2022 at 19:23 | Open on astrodon.social
5 comments
Kelly Lepo

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)

Webb’s Diffraction Spikes: What Are Diffraction Spikes? The top right of the image shows three stars producing eight-pronged diffraction spike patterns. Below this is an image of Webb’s observing side, including its 18 gold hexagonal-segmented primary mirror, struts, and secondary mirror. The smaller secondary mirror is held in front of the primary mirror by three struts. One strut comes from the top of the mirror and two come from the bottom of the mirror. A face-on view shows the portion of the primary mirror blocked by the struts, as well as the hexagonal hole in the center of the primary mirror.
8 Nov 2022 at 19:25 | Open on astrodon.social
Kelly Lepo

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)

Webb's diffraction spikes: How Does Diffraction Happen? There are two boxes. The first box, on the left, displays a star with Hubble’s Diffraction Pattern, which has four-points, two vertical and two horizontal points. An insert shows Hubble's mirrors and struts. The mirror is circular and the struts from a cross shape. The second box, on the right, displays a star with JWST’s Diffraction Pattern, which has eight-points, two vertical, two horizontal, and four diagonal points. An insert shows JWST's mirrors and struts. The mirror is made of of 18 hexagons, arranged into a larger hexagon shape. There are three struts, with two angled at 150 degrees from its vertical strut.
8 Nov 2022 at 19:26 | Open on astrodon.social
Kelly Lepo

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)

Webb's diffraction spikes: Primary Mirror Influence. There are six images, broken into rows of three. The first mirror shape is a circle, which produces a circular image, with a series of concentric light and dark rings. The second shape is a square, which produces a cross-shaped image, with a bright square at the center and four spikes which are filled with alternating bright and dark stripe-like patterns. The third shape is a hexagon, witch produces and image with a bright, hexagon shaped core, which is surrounded by six, equally spaced spikes, which are filled with bright and dark stripes.
8 Nov 2022 at 19:28 | Open on astrodon.social
Kelly Lepo

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)

Webb's diffraction spikes: Strut Influence. Illustrations of struts, the spike created perpendicular to the strut, spikes at detector focus, and the resulting spike pattern. For two struts: two parallel lines are shown perpendicular to the struts. These combine to a single horizontal line at the detector focus, and the resulting spike pattern is two horizontal spikes, one to the left and one to the right of the central point. For four struts: two sets of parallel lines are shown perpendicular to the struts. One set is horizontal (perpendicular to the vertical struts) and one set is vertical (perpendicular to the horizontal struts). These combine into a cross shape at the detector focus, making a four-pointed spike pattern. For three struts: three lines are shown perpendicular to the struts. One is horizontal, one slopes downward and one slopes upward. This combines into three overlapping lines at the detector focus, creating a squished 6-pointed spike pattern.
8 Nov 2022 at 19:30 | Open on astrodon.social
Kelly Lepo

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)

Webb’s Eight-Pointed Stars. A diagram shows how the two diffraction patterns overlap: “Strut Influence" a smaller, squished, 6 pointed star and "primary mirror influence", a larger, evenly spaced, 6-pointed star. Four of the "strut Influence" spikes overlap with the "primary mirror influence" spikes. The the horizontal "strut Influence" spikes do not overlap. The same is true for the vertical "primary mirror influence" spikes. The combination of the two patterns makes an 8-pointed star.
8 Nov 2022 at 19:31 | Open on astrodon.social
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