Fuzzy halos around bright stars with directional orientation

Mike Bagley · Feb 17, 2026 at 08:05 PM

I’ve noticed that I typically end up with fuzzy halos around bright stars, for instance in the bright stars of Orion’s Belt:

📷 Orion's Belt

https://app.astrobin.com/i/182vjp/

They seem to have an orientation to them- near the center of frame, the bright star’s halo is symmetrical, while to the right and left, the halos appear shifted away from center. This image was taken from a pretty light-polluted area using an Askar 71F and an SV220 7nm dual band filter. I processed it using a bunch of the scripts and tools in Siril. Has anyone else encountered this and/or have a suggestion for getting a crisper look?

I doubt that this is due to a spacing issue. This looks to me like a potential problem with either longitudinal color or spherochromatism in your telescope. Longitudinal color is the variation in focal length with color and spherachromatism is the variation in spherical aberration with color. The problem is almost certainly being caused by your dual band transmission filter. It is allowing good focus in one color (red), while the other color (blue) is out of focus. The off-center nature of the halos is merely due to the angle of the beams about the optical axis. I suspect that if you used two separate filters and focused precisely for each filter, the problem will either go away or become so minor that it doesn’t matter.

John

Dave Erickson · Feb 17, 2026 at 09:22 PM

I might add that it could also be a leak in the filters IR blocking band

Dave,

That’s a good thought and something I considered as well but then the halos would happen in the red channel. Since they are happening in the blue, it made me wonder if it could be a leak at the other end of the spectrum, which might occur where the objective is not well corrected.

John

andrea tasselli · Feb 18, 2026, 09:20 AM

The SV220 has an OD of 5 so no chance of an IR or UV leak. Lens ghosting or filters reflections can however produce something like that.

True as well, I have found that the marketing claims sometimes do not match the performance and leaks in both red and blue out of band blockers occur.

Thanks to everyone participating in this thread- it sounds like I should do some experimentation with the filter. Next time I get a clear night, I’ll try shooting the same target with the same set up, completely without a filter and also with just a UV/IR block filter, to see if that mitigates the issue. I’m really hoping that it’s not the result of a nearby streetlight in the general direction that I’m shooting, as there’s very little I can do about that when taking images at home due to the orientation of my small yard and the surrounding trees.

All of my photos (minus an image of Jupiter) are indeed taken with an IMX571 APS-C sensor (a Player One Poseidon-C Pro).