6" Newton at f/3 (Nexus Reducer) - Persistent light artifacts

Starizona Nexus 0.75x Newtonian Focal Reducer/Coma Corrector (NEXUS-075) Apertura 6" f/4 Imaging Newtonian Pleiades Astrophoto PixInsight
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Doversole83 avatar
Doversole83 avatar

Gear:

  • Scope: StellaLyra Carbon 6" f4 Newtonian with Starizona Nexus Reducer (f/3 effective)

  • Camera: ZWO ASI2600MC (APS-C)

  • Focuser: Stock Rack & Pinion (Drawtube protrudes slightly into the light path)

The Problem:

Despite extensive hardware and software troubleshooting, I am seeing a persistent, asymmetrical "glow" or non-linear gradient in my stacked masters. It manifests as a broad "mottled" haze that automated background extraction tools (GraXpert AI, MARS/MGC) fail to model correctly.

I live in a rural area with no street lights.


📷 WBPP.pngWBPP.png📷 image.pngimage.png

What I’ve already addressed:

  1. Hardware: Upgraded the imaging train to M54 adapters to eliminate M42-sized bottlenecks.

  2. Internal Reflections: Blackened the silver rack-and-pinion teeth with Tamiya XF-1 matte paint, added a black shower cap at the back end of the scope and added a black sock around the focuser. I tried rotating the camera by 90 deg. The pattern is moving by not strictly to the angle of the camera.

    📷 PXL_20260501_101731995.jpgPXL_20260501_101731995.jpg

  3. Software: Tested three stacking methods in PixInsight:

    • WBPP with Local Normalization

    • WBPP without Local Normalization

    • FastIntegration (to rule out mathematical "hallucinations")

    • Background extraction using GraXpert or Multiscale Gradient Correction provide the same result.

  4. Flats: I tried a flat pannel and sky flats taken after sunset. No difference.

Observations:

The artifact persists across all integration methods (including FastIntegration), suggesting a physical optical origin. Attached a contour plot of the master flat.
📷 image.pngimage.png

The Question:

Has anyone experienced similar issues? Any clue to resolve this issue please?

Many thanks!!

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andrea tasselli avatar
andrea tasselli avatar

I only had similar issues with my f/4 due to light leaks from both the back and the focuser. Have you tried imaging without the corrector or with other correctors? Besides, your black paint isn’t black enough. I am sure I can flatten your images despite WBPP, So if you want you can post a link to your raws and/or the finally stacked integrations.

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Dirk Bausch avatar
Dirk Bausch avatar

@andrea tasselli's f/4 light-leak diagnostic matches what I've seen on similar setups — that's a useful direction.

@OP — if you're open to sharing the calibrated subs (pre-StarAlignment), I'd be happy to also run them through the pre-stack pipeline I'm developing (thread #236119) as an additional data point. Combined with whatever Andrea produces manually, you'd have two independent approaches on the same data — an expert manual fix and an automated algorithmic one. That kind of side-by-side is genuinely useful for understanding what each method does and where each is strong.

Honest caveat though: as Andrea points out, the reflection component would ideally be addressed in hardware first. BG correction (manual or automated) can mask the symptom but not solve the underlying optical issue.

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Walter Leonhard Schramböck avatar
Walter Leonhard Schramböck avatar

Many focussers (if not most of them) have a gap between the focusser tube and the focusser body. This often is a light leak - even on expensive models. The biggest gap on my rigs is present on a Baader Steeltrack, I bought a wide hair tie and placed it over this gap to reduce light leaking.

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