The circular disc is a rotation stage: the grating, its two inner adjustment slots and the disc turn together as one piece. The detector arm rotates about the same pivot between Angle 1 (95°) and Angle 2 (45°), and slides along its own axis on the focus adjustment ring to set the grating-to-sensor distance. The two outer arcs are the detector mount guides. Dotted lines mark the 45°, 70° and 95° reference angles.