The Baader T-2 Fine-Adjustment Ring, 1.4 mm thick.
Due to manufacturing processes, some T-2 accessories have a small groove at the end of the male thread, allowing thin rings to shift sideways. As a result, the image plane of CCD cameras may tilt slightly, potentially causing stars to appear out of focus in the corners.
Thanks to their snug fit, Baader T-2 Fine-Adjustment Rings cannot slip into the groove found on T-2 accessories, preventing image plane tilt. The rings are made of plastic so that, if they cannot be slipped onto an accessory, they can be easily cut at one point with scissors and gently opened for easy installation and removal.