Off-axis aspheric mirrors are kernel components widely applied in various fields. The off-axis aspheric mirrors with large ratio of sag height to diameter are increasingly employed in many cases for smaller system and better optical performance. Slow slide servo-machining is one of the most currently efficient methods used to fabricate the off-axis aspheric mirrors. However, it is still difficult to machine off-axis aspheric mirrors with large ratio of sag height to diameter. This paper analyzes failure reason in a normal angle, including angle and tool oscillating range. The coordinate transformation machining is proposed to make the machining feasible by reducing the ratio of sag height to diameter. The cutting path is accurately designed by the coordinate transform. Freeform prisms are experimentally machined to prove that the proposed method also works well for complex surfaces.
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