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Coma Corrector

Coma Corrector Equals Round Stars Everywhere [Newtonian Telescope]

A Coma Corrector has never been part of my continuous improvement plan. I have been happy with the progress of my astrophotography. And literally thought nothing of it. At least until a comment was made about my M13 photo on Reddit. It said,

“Very nice , impressive with the hardware definitely have some coma in there and the focus is just slightly wonky which makes those double diffraction spikes on the bright stars.”

From Syscosys on Reddit

Moreover, it was a positive comment, yet I felt the picture fell short of expectations. And after pondering this comment, I realized action was required. I began my research on what exactly is coma. Subsequently that led into a decision to either buy one or buy lenses and 3D print one. Later came failure, rework and success. This project challenged me, so I’ll start at the beginning.

What is Coma?

Coma is short for comatic aberration. It manifests itself as weird egg or fan shaped stars around the outer edges of the photo. In other words, the optical train of the Orion Astroview 6 reflector has imperfections. Light bulb! Did my prime focus modification cause this? More on this later. Let’s look at the picture to help explain coma. Take a look at the circled area.

M13 no Coma Corrector
Telescope & Mount: Orion Astroview 6 with Astroview Mount Camera: Canon EOS T3i Guiding: None Location: Carrollton, VA Sky: Bortle Class 5 Acquisition & Calibration: – Filter: None – ISO 800 – 48 Light Frames @ 90 s – 60 Dark Frames @ 90 s – 60 Flat Frames – 61 Bias Frames – Total Integration Time: 72 minutes

Better yet, here’s a close up view of the stars in that section.

Affect of Chromatic Aberration
Affect of Chromatic aberration

The stars are “egg” shaped and even showing some transparency in the stars. I never really noticed this so much before. Now I’m taking on the challenge to correct this problem.

Coma Corrector Choice

The job of the coma corrector is to force photos of light to meet at the same point. In effect chromatic aberration looks like the first image below. The light misses the intended target focus point acquiring the “fan” or “egg” shape. Accordingly, the second picture is an example of what the coma corrector does. It forces the photons to the correct focal point.

Why one needs a coma corrector
Coma Corrector Model

On the Cloudy Nights message board, found a 3D printed coma corrector. So, hey, this great! I love to 3D print things. But unfortunately I chose to purchase one. Since I’m not up to speed on the lenses I went with the Baader Planetarium 2″ MPCC Mark III Coma Corrector. I’m still going to have to 3D print adapters, so I’m happy there. The reason for this centers around 1.25″ coma correctors only exist in fond memories of a time long gone by. Everything is 2″. So I get my wish and it’s about time I grow up into the 2″ world.

Location of Coma Corrector
Coma Corrector connected to 1.25″ t-ring adapter

The Failure

The thing about failure is to not let it stop your progress. In a word or two failure is a stepping stone to success. In case I spent hours 3D printing sleeves that allowed me to attach the Canon T3i to the coma corrector to the telescope. It looked great, but then I realized the camera was too far away from prime focus. This was going to be troublesome. So I took out spacers and reduced the distance, only to achieve the most hideous photo of the M101.

In compliance with astrophotography censorship rules, I’ve left it out of this post. Definitely, none of the stars had bright centers. In fact, the entire photo had stars with dark centers. Could it be focus, could it be something else. It took me a week of pouring over my steps from 3D printing the bahtinov mask, to mirror extension, to collimation, to coma corrector install to understand where the problem could possibly lie.

Issues Discovered

Let’s go back and look at the photo of M13. If you study the photo you will eventually see a pattern. Because the coma is more evident in one corner than any other part of the image. Let me help you out. It’s circled. At this instant, my thought experiment led me to believe that while collimation was perfect, it was only perfect in 1 direction.

To test this hypothesis, I installed my laser collimator with the window facing the primary mirror. Result, perfect. Then I rotated it 90 degrees right. By and large, the laser was misaligned. I turned 90 degrees right again, same result different location. And the same was true for the final turn. I resorted to aligning the secondary mirror in each of the rotated locations until the alignment didn’t change. Consequently I didn’t touch the primary mirror as I was confident in it’s alignment.

3D Printed Canon T-Ring Adapters
3D Printed Canon T-Ring Adapters

The next issue I went after was room to focus. With the 1.25″ T-Ring Adapter installed, the camera sensor was much further back from prime focus. On the whole, that left very little room to adjust focus before reaching a hard stop on the telescope focus tube. Further more, to solve this problem I moved the coma corrector closer to the camera sensor. I did this by creating a new t-ring adapter that attaches directly to the corrector. Given these points it has a shorter profile which brings the camera sensor closer to the secondary mirror.

Coma Corrector Final Results

Telescope & Mount: Orion Astroview 6 with Astroview Mount Camera: Canon EOS T3i, with Coma Corrector Guiding: None Location: Carrollton, VA Sky: Bortle Class 5 Acquisition & Calibration: – Filter: None – ISO 1600 – 47 Light Frames @ 90 s – 60 Dark Frames @ 90 s – 67 Flat Frames – 61 Bias Frames – Total Integration Time: 63 minutes

Obviously this is a photo I can show. Not the best for other reasons, but that’s not why we’re discussing it. None the less, zoom in and take a look at the stars around the edge. They are more round than I’ve ever seen in any of my previous photos. And yes I looked at all of them. So the question is, what solved the problem? Collimation or Coma Corrector? The answer is both. Ultimately, the collimation fix took care of some of the coma. Looking back on older photos, the coma was always there to a lesser degree. Finally, the coma corrector brought it home. Purchasing a coma corrector will solve your star problem and I expect that the next one I buy will be 3D printed.

Thanks for your time and attention. I wish you the best and look forward to your comments. And remember that the sky is only the limit when your mind is unwilling to fly. Go beyond!

Kevin Francis

Kevin Francis is a Mechanical Engineer by day, amateur Astrophotographer by night who is taking his Google Pixel smartphone camera to new limits.