While we have all been blown away by the first set of images that arrived yesterday from it’s L2 parking orbit, up until recently we’d only seen NASA’s James Webb Space Telescope (JWST) incredible calibration images. One of which highlighted very obvious diffraction spikes – a phenomenon caused by the edges of objects placed in the optical pathway bending the light. In this particular case, they are collectively caused by the vertical support strut holding the secondary mirror, as well as the gaps between the hexagonal mirrors sections, and the hexagonal mirror edge as a whole.
Iris diaphragms also produce similar diffraction spikes, due to the polygonal edges formed by the aperture. How many is directly proportional to the number of leaves used in the iris, with each edge causing two spikes perpendicular and at 180 degrees apart. Consequently, an iris with an even number of blades will produce less spikes than an iris with an odd number, as the spikes from opposite sides will overlap one-another. It is also wavelength dependant, so you’ll likely get slight variations of the spikes depending on what part of the spectrum you’re using.
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Last name, First name [Username]. “Tweet Message.” Twitter, date posted, URL.
NASA Webb Telescope [@NASAWebb]. “Small adjustments, major progress! Having completed 2 more mirror alignment steps, #NASAWebb’s optical performance will be able to meet or exceed its science goals. Now that’s good optics! 😉 https://go.nasa.gov/3KMV1gW #UnfoldTheUniverse Curious about this image? Thread ⬇️” Twitter, 16 March 2022, https://twitter.com/NASAWebb/status/1504121946056888322
管笑天 [@GXT315422]. “I ran the actual NIR selfie through Maskulator. Those thick spikes are mostly caused by the gaps between segments. The horizontal spike is caused by the secondary support(vertical one). The spikes of the other two overlap with the spike of the gap, so are not actually visible.” Twitter, 17 March 2022, https://twitter.com/GXT315422/status/1504312148230475777
Perrin, Marshall [@marshallperrin]. “Correct, it’s both the hexagonal outer perimeter (mostly) and also the segment gaps (which add higher frequency details). Nicely done calculation to reproduce that and figure it out so swiftly.” Twitter, 17 March 2022, https://twitter.com/marshallperrin/status/1504624119844851717
Wikipedia contributors. “Diffraction spike.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 22 Jan. 2022. Web. 9 May. 2022, https://en.wikipedia.org/w/index.php?title=Diffraction_spike&oldid=1067153965
Roman Eisele, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons