Have you ever noticed the 4 little spikes that point away from star images on astronomical photographs, like the crosshairs in a sniperscope? Amateur scope photos as well as big reflectors in professional observatories show these, so does the Hubble imagery.
They are caused by the supporting struts for the secondary mirror in a reflecting telescope. The struts interfere with the light waves from the star, causing the wave to interact with itself. In a point source, like a star, the spikes are visible, in an extended source, like a planetary disk or a nebula, the “spikes” from every point in the image interact with each other, causing a loss of resolution, a “fuzziness” in the image.
In a refracting telescope, which has no secondary mirror suspended in the optical path, there are no spikes, but interference still occurs along the outer edge of the lens.
Diffraction in a refracting telescope manifests itself as a series of concentric circles of light around the central spot of the stellar image. A certain small percentage of the light from the star is diverted to these circles. This has nothing to do with the quality of the lens or precision with which it is ground, it is a property of the wave nature of light itself. All else being equal, the bigger and more perfect the lens figure the less light is diverted to the diffraction pattern. But it is always there. This is why there is a physical limit to the resolution and clarity of a telescope image. The relationship is linear, directly related to the aperture (the bigger the better) and the wavelength of light (the bluer the better). Incidentally, the AMOUNT of light collected, the brightness of the image, is related to the square of the aperture. Making the lens or mirror twice as wide allows you to scoop up 4 times as much light.
The image below shows the diffraction spikes and rings of the JWST, now that the mirrors are all matched to one another. There are 6 spikes, not four, because the Webb secondary assembly has three struts. That’s just how diffraction works, 3 struts gives 6 spikes, 4 struts gives 4 spikes. Go figure. This is why reflectors always have four struts supporting the secondary, not three. Why the Webb was built this way I don’t know; maybe the spikes will be smeared out digitally and removed from the image.
The additional 2 faint spikes visible in an x direction are probably an artifact of the shape of the full mirror assembly. Its not a perfect circle, but is the result of the shapes of the mirror segment hexagons, and the shape or the entire assembly is somewhat like a hexagon. But I’m only guessing.
https://www.cnet.com/a/img/e3UtAse5ZYX4pDpk4m2wt4Z1uVA=/2022/02/25/dad86366-fba2-4d8f-a210-7650cd59435c/webbpostimagestacking.jpg
