In light beam deflection technique achieved by mechanical means, the line image produced, as the polygon rotates, is not straight. A compromise between curvature, working f/number, radius of curvature, and angular deflection has been found.
Sebastiano F. Monaco,
"Field Curvature As A Consequence Of Light Beam Deflection By Mechanical Means," Optical Engineering 21(1), 210173 (1 February 1982). https://doi.org/10.1117/12.7972861