"Wrong" is a strong word. There are compromises in every design. As pointed out by another poster, accurate pole alignment will be the dominant source of error

"Wrong" is a strong word. There are compromises in every design. As pointed out by another poster, accurate pole alignment will be the dominant source of error for any reasonably accurate barn door. The short focal lengths, wide field of view, and relatively short "long" exposures of DSLRs (5 minute exposures will induce thermal distortion on my D60 vs >15 minute exposures for film) common in DSLRs don't require extreme accuracy, especially if stacking the final images. Barn door trackers can have very low error without curving the rod. Modern microcontrollers also allow digital compensation compared to older barn doors that often used analog synchronous motors, and variations on this design (e.g., the double barn door [1]) can provide tracking accuracy at the limit of hobby applications. [1] http://davetrott.com/inventions/double-arm-barn-door-drive/ http://davetrott.com/inventions/double-arm-barn-door-drive/