We study a convex off-axis aspheric mirror which works as secondary mirror in space optical system. The parameters of the mirror are described. In order to test the surface error, the mirror is made up of fused silicon and is tested by the backside transmission type. The shape accuracy while grinding is controlled using coordinate measuring machining testing. The distortion of the measurement is corrected by affine transformation. The ion beam figuring is used for surface finishing and to achieve root mean square of 0.015λ (λ = 632.8 nm).