What Causes Stars to Look Pointy in Pictures?

Credit: John Drummond
The mirror supports on my telescope.
The mirror supports on my telescope

Normally diffraction spikes aren’t noticeable. When viewing objects directly you won’t typically notice them. But with long photographic exposures they generally show up.  Even professional telescopes have them in many of their images. They generally aren’t a problem for research, and the advantages of mirror telescopes vastly outweigh the minor inconvenience of diffraction spikes.Mirror-based telescopes are easier to make, and because they reflect light they can be made shorter. Light can be focused from a large back mirror to a smaller mirror, which then focuses the light onto an eyepiece or camera for viewing. One downside of this type of design is that starlight has to pass the smaller mirror before reaching the large back mirror. As it does, the supports for the mirror cause the light diffract. It’s the diffraction pattern that causes stars to appear spiky, hence the term diffraction spikes.

But the main reason we see diffraction spikes so often is that astrophotographers often use them to artistic effect. They transform a bright point of light into a wondrous stellar image.

Brian Koberlein is an astrophysicist and physics professor at Rochester Institute of Technology. He writes about astronomy and astrophysics on his blog "One Universe at a Time," as well as on Google+. You can follow him on Twitter @BrianKoberlein