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Why do the spikes that shoot out of stars form perfect crosses?

Category: Space
Published: November 16, 2015

The crossed spikes that you see in some images of stars are not actually parts of the stars. They are imaging artifacts that are created by the telescope itself and are called diffraction spikes. Certain telescopes have a large primary mirror that focuses the incoming beam of light onto a secondary mirror or a sensor that is held over the primary mirror. The secondary mirror diverts the light out of the telescope so it can be seen or further processed. Or, alternately, a sensor held above the primary mirror converts the image to an electrical signal that is delivered to a computer.

stars with spikes
On the left is a typical image of stars with crossed diffraction spikes. On the right is an image of the type of support bar configuration that creates these crossed spikes. Note that the image on the left was not taken using the telescope on the right. Public Domain Images, source: NASA.

The key point is that the secondary mirror or sensor must be held in place over the primary mirror by support rods which are in the way of the incoming light. These support rods are also called struts or vanes. Some of the starlight that enters the telescope and heads toward the primary mirror skims past the support rods and gets deflected slightly in the process. This deflection of light that skims past an obstruction is called diffraction. The diffraction caused by the support rods ultimately shifts light in the final image to places where it did not originally exist. For stars and other bright point sources of light, this shifted light pattern takes the form of radial spikes. When the support rods of a telescope's secondary mirror are built in a nice, symmetrical cross pattern, the diffraction spikes in the image of the star takes on the same cross pattern. A collection of support rods or vanes in a telescope is called a spider.

The book Practical Astrophotography by Jeffrey R. Charles states,

A spider such as that used to support the diagonal mirror of a Newtonian telescope also introduces diffraction. Each diffraction spike is oriented perpendicular to the boundary of the vane causing it, so a spider having two straight vanes which are oriented at a right angle to each other will produce two intersecting diffraction lines (or a total of four spikes) having the same perpendicular orientation to each other. Since the spikes are from diffraction, they will appear on both sides of the star image even if a spider vane is on only one side of the central obstruction. For example, a three vane spider in which the vanes are 120 degrees apart will produce a star image having three interesting diffraction lines, for a total of six spikes.

Because the diffraction spikes in the images of stars are created by the support rods, a telescope that has no support rods will create images without diffraction spikes. For instance, a telescope that uses only lenses does not need support rods and therefore creates images containing non-spiky stars.

diffraction spikes
The top row shows various telescope support rod configurations and the bottom row shows the corresponding star image diffraction spikes. Public Domain Image, source: Christopher S. Baird.

Diffraction of the incoming light happens not only along the support rods, but also along the edge of the aperture that lets the light into the telescope in the first place. The edge of the aperture constitutes a sharp edge that the incoming light must pass, therefore it causes diffraction. The apertures of most telescopes and cameras are circular. As a result, apertures typically create diffraction rings (called an Airy pattern) rather than diffraction spikes. Often, the diffraction rings are very faint and just make the image slightly blurry. But with sensitive equipment and long exposures, the diffraction rings caused by circular aperture can be readily seen.

If the aperture is not circular but has some other shape, then both rings and spikes can result from just the aperture. For example, many cameras have diaphragms, which are structures that allow the photographer to adjust the size of the aperture. These diaphragms typically create apertures that have polygonal shapes, such as pentagons or hexagons. Such polygonal apertures also cause diffraction spikes. Diffraction spikes seen in images taken by lens-based cameras are therefore not caused by support rods but by the non-circular aperture. In contrast, telescopes usually have circular apertures and therefore create images with diffraction spikes caused by the support rods.

Topics: diffraction, diffraction spike, spike, star, telescope