Definition
Perpendicularity error occurs when the index mirror is not square to the frame, detected by viewing the arc's reflection for a step. Side error arises when the horizon mirror is not perpendicular to the frame, shown by a horizontal displacement of a star image. Collimation error is a telescope axis not parallel to the frame and is rare in modern instruments. These are removed in order, since adjusting one can reintroduce another; whatever remains is index error.
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