All point sources (i.e., stars) are affected by diffraction spikes, but brighter sources have larger spikes, which is why diffraction spikes appear obvious around the brightest stars but are unnoticeable in fainter ones. You can get many types of diffraction spikes, depending on the shape of your mirror (or aperture) and the number and orientation of secondary mirror struts. The two small horizontal spikes are from the remaining strut. The telescope’s design is such that the spikes from the mirrors and the spikes from two of the spider struts combine, creating the six larger spikes. (Hubble’s primary mirror doesn’t cause the same effect because it is a single round piece of glass.) Incoming light diffracts when it encounters the edges of each of JWST’s non-round segments. JWST’s spider has only three struts, but in this case, not only these struts are diffracting incoming light so are the 18 hexagonal segments of JWST’s primary mirror. This is where things get a bit more complex. For example, in Hubble Space Telescope images, we typically see four spikes at right angles to each other because Hubble has four metal arms holding its secondary in place.īut you’ll notice that images from the James Webb Space Telescope (JWST) have six large spikes and two small ones. This interference pattern appears in the shape of the orientation of the struts these are the diffraction spikes we see. That secondary is held in place by thin metal struts (collectively called the spider), and these struts are largely the cause of diffraction spikes.Īs incoming light waves diffract (bend slightly) around the edges of the struts, the waves begin to overlap each other, causing an interference pattern like ripples overlapping in a pond. Normally, a high-quality lens, wide open, will not produce. Light hits the primary first and reflects onto the secondary before being reflected back through a hole in the primary mirror to the telescope’s focus (where the camera sits). Bright stars stand out better in a picture if they are surrounded by diffraction spikes. However if your images are widefield with hundreds of stars then spikes can really reduce the viewing pleasure. Reflecting telescopes have two mirrors: a large primary and a smaller secondary. I love spikes in SOME images (The Seven Sisters) and other asterisms. The diffraction spikes so familiar to us in space images are an intrinsic property of the telescope. Why do they appear as two crosses at 45 degrees to each other? I learned in October 2022’s Breakthrough that the spikes of stars in high-power photos are called diffraction spikes. Credit: Astronomy: Roen Kelly, after Cmglee & Pete Lawrence/ Wikimedia Commons The patterns produced depend on the shape of the telescope’s mirror and the number of spider struts. Many imaging enthusiasts like the look of spikes on bright stars, and they add them by placing some kind of mask in front of the aperture to mimic the effect of a Newtonian reflector’s spider vanes. Diffraction spikes are caused by interference due to the wave nature of light. The diffraction spike sets apart a star in the Hubble Ultra Deep Field.
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