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

@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.

I made the same experiance as Walter. Also with a Baader Steeltrack and I solved it in same way. Since then I had no problems with a light leak from there.

📷 1000035541.png

Take darks during daytime (but not too bright a daytime!) will soon tell what is going on…

That’s good, it means no external light leaks. The bad news is that it’s likely an internal reflection of sorts in the optical train.

andrea tasselli · May 15, 2026, 04:50 PM

Are you sure those images have been calibrated at all. Because either they aren’t or the calibration is wrong. Maybe supply the raw uncalibrated together with master flats & darks

Thanks again for having a look.
I added the uncallibrated raws in the folder as well as the masterflat and masterdark.

So you think it’s not an external light leak, but more of an internal reflection? Where could that come from? Could the Nexus be faulty? I haven’t tried the scope with another corrector or no corrector. Next clear night I suppose I could do that.

Here is the best I could do so far. I can do better if I crop the borders but let’s leave that for later.

📷 image.png

andrea tasselli · May 15, 2026, 06:03 PM

Also, parasitic lights could be another cause. Your gain is set to 0.243 for the light and the offset to 50. I can’t check the offset of the master dark and flat but the gain is the same. Are you using a dew shield at all?

I use an ASIAIR. You dont set the offset, just the gain. It should be gain 100.

I use a dew shield (it's wet England!).

I have tried 2 different dew shield, same result.

The master flat I shared was done with a flat panel and dark flats

I also tried with some flats taken after sunset, they were longer (4s) and I didn't use any dark flats. Again same results. I don't usually tend to use darks with this camera but to find the origin of the issue I started to take some anyway.

Doversole83 · May 15, 2026, 06:11 PM

I have tried 2 different dew shield, same result.

The point with the dew shields is that you must use the same one for the flats that you used for the lights. Again, going with the mantra that with flats, everything must be the same except for the gain.

PXL_20260515_150407432.jpg

Would the focuser tube intrusion inside the scope explain these artifacts? Do I need to shorten the focuser?

Doversole83 · May 15, 2026, 10:42 PM

PXL_20260515_150407432.jpg

Would the focuser tube intrusion inside the scope explain these artifacts? Do I need to shorten the focuser?

Usually what you’ll see emanating from each bright star will be a broad fan of light 180 degrees opposite each other, one of which lines up with the focuser position. It’s important to remember that anything that intrudes on that path will cause unwanted diffraction effects. Unfortunately, to get enough back focus most Newtonian astrographs tend to use tubes that are too small in diameter so it’s hard to avoid the focuser issue without very careful design. It’s a dance between tube diameter, focal plane position, secondary size and field illumination.