What is Comatic Aberration (Coma) in Telescope?

Comatic Aberration, also called Coma, is an off-axis optical aberration, meaning it has a greater effect as it nears the edge of the field. This results in a blurred tail around the object you’re viewing.

What Causes Comatic Aberration?

Coma is caused by the deviation of light rays in lenses or mirrors. This causes rays of light entering a lens to be brought into focus at different positions. The difference in refraction produces an aberration that resembles the tail of a comet

What are the Types of Comatic Aberration?

There are two types of comatic aberrations: negative and positive. When peripheral rays produce the smallest image, the comatic aberration is considered negative. This results in a tail pointing away from the center of your field of view. Positive comatic aberration occurs when peripheral rays are focused further down the axis, with greater magnification. A positive coma causes the trail to appear towards the center.

Which Telescopes Experience Comatic Aberration?

Comatic aberration is one of the most common optical aberrations, especially in reflector telescopes. Telescopes like the Newtonian, Schmidt-Cassegrain and Maksutov-Cassegrain all suffer from a degree of coma. Some designs, like the Ritchey-Chrétien, eliminate coma, but instead experience astigmatism. Because coma is most problematic when imaging, most astronomers prefer to deal with astigmatism. 

How to Fix Comatic Aberration

Selecting a telescope like the Ritchey-Chrétien allows you to eliminate coma. If you already have a telescope, and are experiencing comatic aberration, there are still ways to reduce or eliminate its effects. The simplest is to keep the object you’re viewing close to the center of the lens. Because comatic aberration worsens as you near the edges, this allows you to minimize the effect without purchasing any equipment. You can also purchase higher quality lenses or mirrors, designed to reduce or eliminate this aberration.