What is Astigmatism in Telescopes?

Astigmatism is an off-axis optical aberration caused by the obliquity of focal planes. This results in rays of light meeting at different focal points, distorting the image.

What Causes Astigmatism?

Astigmatisms are caused by optical defects or rays of light hitting the telescope obliquely. Telescopes have two perpendicular planes; the tangential and the sagittal. Astigmatism occurs when these planes have different focal points. This creates two focal surfaces, which causes light to focus on one axis ahead or behind the other. 

Which Types of Telescopes Experience Astigmatism?

While most telescopes are designed to reduce astigmatism, it can still occur in any optical device. This is because any eye, lens or mirror is prone to damage or error. Some telescope designs, like the basic Ritchey-Chrétien, are expected to suffer a degree of astigmatism. This is because they are designed to eliminate coma and spherical aberration instead. Astronomers using these designs prefer astigmatism over coma because it still allows you to find accurate positional measurements. 

How Does Astigmatism Affect Astronomical Viewing

Astigmatism appears differently depending on the quality, magnification and focus of your telescope. When stars are in-focus, they appear as a blurred cross. When they are out of focus, they will resemble an ellipse. With astigmatisms in the tangential plane, light is stretched horizontally. Sagittal astigmatisms appear to stretch light vertically.

How to Fix Astigmatisms in a Telescope

Astigmatisms can have a number of causes, ranging from your eyeball to optical limitations. In refractor or catadioptric telescopes, this aberration is typically caused by a defect or limitation in your telescope’s optics. To reduce the aberration, you typically need to purchase a new lens.

In reflector telescopes, astigmatisms have a number of potential causes, typically residing in the mirrors. One common cause is pinched optics, which occur when manufacturers over-tighten the mirror clips. While this ensures proper positioning during shipping, this can bend the mirror just enough to cause astigmatism. To resolve this, you can loosen the screws enough to relieve the pressure. If the aberration persists, you may have to replace the mirror.