It looks like the flocking material around the edge of the lens group does fix the issue - at least with my limited testing this evening. I decided to remove the flocking material and imaged a patch of the sky that exhibited rainbows, then re-added the flocking material and imaged the same exact patch of sky to see if the rainbows disappeared.
In the screenshot below, you can see on the left two faint rainbow artifacts on the single 60-second frame captured with no flocking material. Then on the right, the same exact framing and camera rotation, but this time with the flocking material. These are just single 60-second frames with an ABE in PixInsight. There are no rainbows visible!
I actually added a narrower circle of flocking material this time (~5.5mm), and that appears to be enough. I made sure it was black on both sides by sticking two bits of flocking material back to back, just in case the white backing paper causes any side effects due to its reflectivity. Obviously time will tell whether this definitely resolves the issue, or whether any other bright stars in different parts of the field of view still cause rainbows.
Have you had a chance to test something similar?
I didn't notice the rainbows in my images, but what I do see and it is the same in your images are the dark red rings. They happen with or without a filter.
Did anyone notice these and/or find a solution for them? There is a big one that touches the top and bottom edges and then concentric ones outside and inside less visible.
The train is Baader UFC, Baader tilter, Baader rings, RASA adapter, everything S70 up to M48 adapter to the ASI2600 camera. The filter holder is the first mounted thing on the camera.
LE: I've tried the 3D printed part, although I don't have rainbows, and it looks like it removed the red-brown rings. At least, I cannot see them anymore.
Edited by sigxcpu, 02 November 2023 - 07:59 AM.