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Diffraction Spikes in Astronomy Pictures

-- cos-mick | September 19, 2021

Question:

Dear Mr. Mangum,
I was looking at the hubble pix on NPR.org today 9/16/21 of the “two views of the same area in the star-forming nebula NGC 2174” and I noticed that the stars all seem to have North-South/ East-West right angle brilliance projections.
https://www.npr.org/2021/09/16/1036600340/nasa-is-launching-a-new-telescope-that-could-offer-some-cosmic-eye-candy

Q: Is this view common or photo enhancement or Lens to human perceived object reflectivity?
Q: If common/universal, why 90 degrees?
Q: Is the “tilt angle” of the 90 degree related to axis rotation?
-Thanks

-- cos-mick

Answer:

What you are seeing in these “spikes” emanating from bright stars in these images is due to the optics of the telescopes used to take the pictures. The mirrors in optical telescopes are often mounted using structures that partially interfere with the incoming light from stars. Those structures cause “diffraction” of the light around them, which results in the “spikes” that you see emanating from the bright stars in astronomical images.

-- Jeff Mangum

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