In a study published in Optics and lasers in Engineering, a research group led by Prof. ZHANG Xuejun from the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) proposed the concept of wavefront optical spacing (WOS), and a cat-eye computer-generated hologram (CGH) test method.

With the development of astronomical telescopes, there are higher requirements for optical systems. The most important factors for high precision optical systems are surface figure quality and geometric parameters.

The interferometric compensation test is widely used in optical surface figure testing. However, the problem is that the interferometric compensation test cannot simultaneously test the surface figure and geometric parameters of freeform surfaces with high accuracy.

The researchers proposed the WOS concept to describe the spacing geometric parameters under interferometric compensation test. The WOS concept is the distance between the wavefront at the ideal design position and that at real surface testing position along the wavefront propagation direction. The coupling relationship between WOS and surface error was analyzed, especially under freeform surface conditions.

Besides, the researchers proposed a cat-eye CGH test method combining the cat-eye principle and CGH interferometry for accurate WOS measurements. A cat-eye zone to original CGH compensator was added. The cat-eye zone modulates the original divergent spherical wavefront to a convergent spherical wavefront, and the convergence point is at the cat-eye position.

This method can achieve high accuracy testing of surface figure and WOS simultaneously under interferometric compensation test conditions. For a freeform mirror with 2139.854 mm curvature radius and 320 mm aperture, the test accuracy is 10.2 ± 4.3 μm (P=95%). And the method is not limited to the freeform surfaces, but includes the testing of sphere and the aspheric.

The high accuracy test of the WOS can be applied to guide the optical system redesign and tolerance balance. The cat-eye CGH method contributes to a high testing accuracy with a simple configuration.