What is Coma Aberration?Before we start talking about how to remove or even avoid coma in your astrophotography images, lets first talk about coma aberration. It is an imperfection in some lens designs that result in stars appearing distorted, sometimes looking like they have a little fuzzy tail, like a comet.
Depending on what lens you use, you might have noticed the stars near the corners are not perfectly round. They might appear elongated, even if you’re using short shutter speeds.
Sometimes the coma is very noticeable. Below is an example of the coma on my Canon 50mm 1.8 lens; The “nifty fifty.” This type of coma is often called “angel wings.”
How to Prevent Coma?
The easiest way to prevent coma aberration is to stop down your lens. If a lens shows coma when shooting wide open, choosing an f-stop one or two stops down from wide open often removes all or nearly all coma aberration. The problem with this approach is you lose precious light! Maybe a better performing lens is what you need?
Lens aberrations are one area where your gear does matter. Different lenses will handle coma differently. Does this mean you need expensive lenses? Not at all! Rokinon, also called Samyang depending on where you live, manufactures some excellent low budget high-performance lenses. The Rokinon 14mm F/2.8 lens sells new for $300, and the Rokinon 24mm F/1.4 new for $500. As a trade-off for low coma and a fast aperture, these lenses are manual focus and manual aperture. They also have some consistency issues, so be sure to check your copy before the return period is up.
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How to Fix Coma in Your Astro Photos
You might not have a low coma lens, but that doesn’t mean you have to live with the coma in your images. There are a few methods to get rid of that nasty looking coma.
The first method to remove the coma is to crop it out. If you crop out the coma in a single frame, you’re probably going to lose too much detail and resolution. Instead of cropping a single frame, shoot a panorama! Allow at least 30-50% overlap in your images and photograph a wider area than you need. When you combine the shots in post-processing, the software will use the best parts of each frame, removing the coma. You’ll still have some in the edges, but since you shot a panorama, you’ll have plenty of room to crop out the coma.
Another simple method to get rid of coma is to remove it in Adobe Photoshop. Tyler Sichelski, from Lonely Speck, made a YouTube tutorial on how to do just that. Tyler is a photographer based in Arizona. He specializes in landscape, Astro, long-exposure, and horse photography. Check out his work on his website, https://www.tyskiphoto.com. Don’t forget to check out https://www.lonelyspeck.com for more tips and advice as well!
How to Remove Coma in Photoshop:
In Tyler’s video, he walks through how to create a brush tool that will remove the coma. A screenshot of the brush design is below. Click here to watch his full tutorial.
To use this brush, select the clone stamp tool, select the coma corrector brush design, sample an area near the star you want to correct, move over the stretched star and use the clone stamp tool. The black areas on the brush will allow the selected information to be pasted over top of the stretched star leaving the white area unaffected.
Tip: to adjust your brush size use the left and right bracket keys, [ ].
Don’t have Photoshop?
Why even worry about Coma?
Coma can give the misconception that your image isn’t sharp. If you have a lens that has decent coma performance, you might not notice much difference, but the devil is in the details. We spend thousands of dollars on our gear and invest countless hours and days into getting a few images, so I encourage you to give it a shot!
If you plan to add this to your workflow, let me know! If you have a different way of dealing with coma, let me know in the comments below.