Avoiding comatic aberration during night sky photography
One issue photographers have to contend with when photographing the stars at night is a comatic aberration from their lenses, also known as a coma. Coma can cause pinpoint stars to appear distorted, particularly at the edges of the frame.
Do not confuse comatic aberration with chromatic aberration, a different optical issue.
What is comatic aberration?
Comatic aberration is an optical defect present in most photography lenses. When light strikes a lens at an off-axis angle, the rays bend and converge at different points. The result is that stars don’t appear as sharp pinpoints in your image but are distorted, with blurred, comet-like shapes.
When photographing the stars, you will see this more on the edges of your frame. In the center of the frame, stars will appear as sharp pinpoints because their light rays hit the lens directly on its axis. But at the edges of the frame, starlight enters the lens at an off-axis angle, and you may see oddly shaped or blurry stars. The coma effect increases as you widen your aperture (lower f/stop numbers).
How to fix comatic aberration?
Night sky photographers try to maximize the light they gather and minimize digital noise in their images. This means they want to use the widest aperture (lowest f/stop number) possible. However, using very wide apertures can lead to unwanted coma and distorted stars in the image.
Because lenses show more coma at their widest apertures, the easiest solution is to stop down the lens to a smaller aperture (higher f/stop number). This will seem counterintuitive. You may have bought a wide aperture f/1.4 or f/1.8 lens for star photography, yet you don’t get to use its widest aperture to gather the most light and reduce digital noise.
Each lens design handles comatic aberration differently. Some lenses show strong coma effects when photographing stars, while others manage the aberration better. To find the optimal aperture for your lens that balances light gathering with reduced coma, you should take a series of test shots of the stars while varying the aperture.
Start at the widest aperture, like f/1.4, then widen the aperture in increments to f/1.6, f/1.8, f/2, f/2.2, and f/2.8. On a large screen, evaluate each photo by looking at the stars in the far corners of the image. You’ll see that as you use smaller apertures, stars in the corners become sharper. While this varies by lens model, photographing at f/2 or f/2.2 often results in better-looking stars than f/1.4 or f/1.8.
Find a balance
While every night sky photographer wants to minimize digital noise and gather the most light with the widest possible aperture, this needs to be balanced with reducing comatic aberration to ensure the best image quality. Comatic aberration is nearly impossible to remove during post-processing, and it is easier to remove digital noise than to fix coma, so it is often best to photograph using a slightly smaller aperture on your lens.