Diffraction spikes

Stars don't actually have spikes, but that's how we see them thanks to bending light

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When it comes to astrophotography, stars can take many forms. They may appear as tight and bright pinpricks in some images or pointy crisscrosses with elongated lines radiating from their centre in others. The latter are called diffraction spikes and, depending on the photographer, they're either a blight on an image or a pleasing aesthetic.

Whatever your opinion, it's worth understanding what causes diffraction spikes. They are not the result of poor focus or your camera sensor, but rather down to how light from a star interacts with the internal mirrored structure of a reflecting telescope.

Newtonian, Cassegrain and Ritchey–Chrétien telescopes typically produce these spikes when incoming light hits the primary mirror and the vanes (or 'spider') supporting the secondary mirror, causing it to bend or 'diffract'.

This diffraction redirects the light into a particular pattern determined by the shape and numbers of the supporting vanes. The shape of the primary mirror also has an effect.

Diffraction spikes can also appear as a result of light interacting with camera lenses. In these cases, it's not the vane causing the diffraction, but the aperture blades used to control the amount of light entering the lens. The higher the f/number, the smaller the aperture; this, in turn, makes the opening less circular too, meaning that more incoming light is diffracted and the more prominent the spikes become.