Rainbow halo artifact on bright stars with Newton and ASI533MC

Reflectors Artifacts Help Needed Troubleshooting
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TiagoAstro avatar
TiagoAstro avatar

Hi everyone,

I’m trying to understand a strange artifact on my setup and wondered if some of you already experienced this.

Setup:

  • SkyWatcher 150/750 (with Backyard Universe spider and full black flocking inside)

  • SkyWatcher coma corrector

  • ZWO ASI533MC Pro

  • No filter

On bright stars, I get a weird rainbow/green halo or ghost reflection.
What’s strange is that the artifact depends on the star position on the sensor: when the star moves across the field, the halo changes position too.

It even appears on very short exposures (~3s), so it does not seem related to long exposure blooming.

I’m wondering if this could come from:

  • internal reflections between the sensor window and coma corrector,

  • absence of UV/IR cut,

  • the coma corrector itself,

  • slight dew,

Has anyone seen something similar with a Newton + ASI533MC setup?

Thanks!

📷 IMG_0403.jpgIMG_0403.jpg📷 IMG_0401.JPGIMG_0401.JPGnamely, there is no dust and no impact on the optics.

Well written Respectful Concise Engaging
Werner Stumpferl avatar
Werner Stumpferl avatar

use something like this in front of your main mirror:

https://www.teleskop-express.de/de/teleskop-zubehoer-5/weiteres-teleskopzubehoer-63/backyard-universe-blendenring-sw150-cnc-gefraest-16723

TiagoAstro avatar
TiagoAstro avatar

I already use one…

Werner Stumpferl avatar
Werner Stumpferl avatar

Hmmm, thats interesting. I had the same issue until I mounted one.

Is your secondary mirror blackened ?

TiagoAstro avatar
TiagoAstro avatar

No it’s not. Should I ? I used to have a Kepler corrector and never had any problems (other than ugly stars)

Werner Stumpferl avatar
Werner Stumpferl avatar

I´m not sure if this helps for this problem but it helps for some kind of reflections that you can avoid with this.

Tony Gondola avatar
Tony Gondola avatar

It’s probably internal reflections being caused by your coma corrector. This is not uncommon with the giveaway being that the reflection moves with different placements of the star in the field. A diffraction artifact wouldn’t do that.

Well written Helpful Respectful Concise Engaging
bigCatAstro avatar
bigCatAstro avatar

I agree with @Tony Gondola that it’s probably an internal reflection caused by the coma corrector. I would have the same sort of problem when I was first starting out imaging years ago with a Celestron C8N and I couldn’t figure it out until I removed the corrector.

Well written Concise Engaging Supportive
TiagoAstro avatar
TiagoAstro avatar

Okay, thanks. What other solution is there besides removing the corrector? As many people have said, they have this corrector and haven't had any problems with it.

bigCatAstro avatar
bigCatAstro avatar

TiagoAstro · May 20, 2026 at 02:13 PM

Okay, thanks. What other solution is there besides removing the corrector? As many people have said, they have this corrector and haven't had any problems with it.

In reality, no two pieces of equipment are exactly alike, so there could be defects with your specific corrector. I would at least test removing it and rule it out as a cause.

Well written Concise