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