Fiducial free correction of mapping distortion in null test of aspheres and freeforms

Aspheres and freeform surfaces are well measured with null optics but the transverse coordinates on the test surface are not linear with the pixels on the imaging plane. Such a mapping distortion must be corrected before the measured surface error map is used for corrective figuring process. Conventional mapping distortion correction involves tediously finding the coordinates of a sequence of fiducial marks on both the test surface and the imaging plane. This paper presents a pure ray tracing method without use of fiducial marks by introducing a dummy spherical surface between the point source and the null optics, acting like a virtual reference sphere (RS). It is linearly mapped to the imaging plane with the linear scale easily calibrated. On the other hand, the virtual RS is exactly mapped to the test surface by tracing a bundle of dense rays. This virtual surface thus bridges the gap between coordinates on the imaging plane and those on the test surface. Two experimental examples of null test of on-axis and off-axis aspheres demonstrate the effectiveness of mapping distortion correction.