What Is Coma?
Coma or comatic aberration is an optical aberration in lenses where the image appears as a comet shape. It is caused by defects in the lenses where off axis light is not focused to one point. Instead, it has multiple circles that diminish such that the result looks like a comet.
All lenses have coma to some point or another. For astrophotography it is a problem that can affect the overall image and ruin the shape of the stars.
In this example, I shot M45 with the Orion CT80 and had coma all over the image plane. Notice that the “tail” is always in the same direction and is clearly not a star trail. A star trail would be the same width and look like a streak. You can clearly see that the stars are comet shaped.
How To Test For Coma
You can test a couple of different ways. For one, you can simple photograph some stars and look at the shape over the entire field of view.
Another method is to be a more methodical about it and use a test apparatus. I have designed and built a simple tester using an LED flashlight and a custom cap.
In FreeCAD I created a solid cap that slips over my flashlight. I printed this out and then drilled a small hole. I used the smallest drill bit I had.
After this cap was printed, I drilled a small hole in the center. I used the smallest drill bit I could find. Next, I applied several layers of blue painter’s tape to the inside of the cap. This further reduced the amount of light. Without the tape, the light is so bright that it blew out the image. The goal here is to simulate a distant star so I’m trying to get a small, dim, and round light that looks like a star. With enough layers of tape, this is possible.
Next I put the light on the fence rail and set my equatorial mount about 80 feet away. At dusk, I started the test. It needs to be fairly dark so that the dime light from the tester is just barely visible.
My first shot was wide open. When I zoomed in the coma could be seen but it was not as obvious as it is with a real star.
I then took subsequent photos with one stop down and then 2 stops down. At each interval, the amount of coma was reduced.
Test Using Real Stars
As soon as it got dark I tested again but this time on real stars. At 400mm, the lens showed significant comatic aberration when shot wide open. So much so that the entire image would be ruined if no interventions was made to reduce the coma.
I repeated the same test as I did earlier by stepping the lens down one stop at a time.
At even one stop down, the coma was dramatically reduced.
At 2 stops, it’s was barely noticeable.
How To Reduce Coma
Stepping down the lens is the most effective way of reducing coma. Unfortunately, this is not what astro photographers want to do. We would like to shoot wide open in order to get the most light possible.
Telescopes or lenses that have reduced coma generally are expensive. So you are left with few options if you are on a budget.
So why does this work?
Well, in refractors and reflectors, the coma is caused by imperfections in the lens curvature or the mirror. This imperfection is most noticeable away from the center of the lens. As off axis light strikes the outer part of the lens, it produces a larger image that is shifted. This shift is why comatic aberration appears as a comet, where it gets larger away from the true center of the star. By stepping down the lens, less of the problem area is used in the final image since we are blocking the light at the edges of the lens.