An off-axis cyclic radial shearing interferometer (OCRSI) to test a centrally blocked transient wavefront is proposed. Based on the standard cyclic radial shearing interferometer (CRSI), the OCRSI consists of a beam splitter, two folding mirrors, and a Galilean telescope. With the same but reversal tilt introduced to the two mirrors in OCRSI, the shearing interferogram can be obtained even when the central part of the test aperture is blocked. An improved wavefront retrieval method for OCRSI is employed, and a method to obtain the laterally sheared amount between the contracted and expanded beams is proposed. Numerical simulation and comparison experiments with a ZYGO GPI interferometer demonstrate that the OCRSI exhibits high precision and nice repeatability.
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