Hydrogen Orange

Thanks, Andrey. It seems the issue isn't with collimation, but with processing. I overcomplicated the histogram in PIPP. I've switched to PixInsight, where processing is much more convenient.

No!.. It's precisely in the collimation! Your sensor is most likely slightly tilted or shifted relative to the optical axis. The most probable cause is a tilt in the focuser or, since there essentially isn't one in a LUNT or PST, in the camera adapter tube. Because of this, the vignetting shifts across the frame, distorting the illumination pattern.

By the way, yes, I might not have tightened some parts all the way. It arrived fully assembled. Thanks for the tip, I’ll check and reassemble it more carefully.