Star shape issues after putting in main mirror mask - Forums

Topic starter

Dec 27, 2025

Astrobiners,

I have a CarbonStar200 to which I added a primary mirror mask. Having added the mask, my star shapes aren’t what I would expect and I’m soliciting thoughts on possible root causes.

Attached are two images - these are staged and simply screen stretched. The first image was taken pre meridian flip, the second post meridian flip.

📷 PreMeridian.jpg📷 PostMeridian.jpg

As you can see, pre-meridian flip there is quite a bit of flaring around the large star in the lower right portion of the image. Stars on the left portion of the image look more like what I would expect. Post-meridian flip shows the same features. The large star in the upper left looks ok to me, stars on the right have flaring associated with them. I initially had too thoughts:

1) After installing the mask my collimation was way off, to get the system back in collimation on of the adjustment screws is nearly bottomed out. So I wonder if this is stressing the main mirror cell and causing the issue I’m seeing.

2) I also noticed that the flaring is in the same position as the focuser. While the focuser screws are anodized black they’re pretty shiny. So, I’m wondering if it could be a stray light issue.

Any thoughts would be greatly appreciated. Thanks…

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andrea tasselli

Frequent contributor

Dec 27, 2025

Likely because the ring isn't smooth and regular enough and/or isn't covering the clips enough.

Tom Boyd

Topic starter

Dec 27, 2025

andrea tasselli · Dec 27, 2025 at 07:12 PM
Likely because the ring isn't smooth and regular enough and/or isn't covering the clips enough.

Thanks Andrea. I’ll check both. Much appreciated…

Tom Boyd

Topic starter

Dec 28, 2025

So, followed up on Andrea’s suggestion. No luck - mask is well positioned and smooth. In addition I did a couple of other tests. My first thought was that the artifact is being caused by a nearby street light. So I changed shooting locations. No luck artifact still there. I then thought rotate the image train by 180 degrees. Artifact rotates with image train. So, my feeling now is that it’s something with the coma corrector or camera. I eliminated the filter wheel and filters as the artifact shows up on multiple filters. I’m going to tear apart the image train tomorrow…

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bigCatAstro

Dec 28, 2025

Tom Boyd · Dec 28, 2025 at 01:20 AM
So, followed up on Andrea’s suggestion. No luck - mask is well positioned and smooth. In addition I did a couple of other tests. My first thought was that the artifact is being caused by a nearby street light. So I changed shooting locations. No luck artifact still there. I then thought rotate the image train by 180 degrees. Artifact rotates with image train. So, my feeling now is that it’s something with the coma corrector or camera. I eliminated the filter wheel and filters as the artifact shows up on multiple filters. I’m going to tear apart the image train tomorrow…

That makes sense, if it rotates with your imaging train, then I would also be looking at the corrector and then the camera.

andrea tasselli

Frequent contributor

Dec 28, 2025

That's interesting because it is likely it was there all along, just masked by the clips' spikes. Go figure…

Craig Towell

Dec 28, 2025

Looks like the off axis light from one side is being affected by something - looking at how spiky the asymmetric flaring is I’d guess it’s something with a rough edge. Could try removing the camera (leaving everything else in place) then pointing the scope at the daylight sky and looking through the image train to see if you can identify any reflections/obstructions.

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Tom Boyd

Topic starter

Dec 28, 2025

Thank you for the analysis and suggestion. I will give that a try…

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