With the development of optical machining and testing, the gap between freeform surface fabrication and design has been filled. Based on the freeform surfaces’ extraordinary feature for balancing asymmetric aberration and compressing system volume, an volume-compression optical design method for off-axis, three-mirror system is presented. Based on the guide of Nodal Aberration Theory for balancing asymmetric aberration, some optimization rules and tricks are shared during the optimization process. In the end, a highly-compact optical system with wide field of view, large entrance pupil and small F number is shown, the image quality is close to diffraction limit at Nyquist frequency.
From the nodal aberration theory, aberration nodes will move to other locations of field of view when the optical element is decentered or tilted in rotationally symmetric system, which will lead to uncorrected off-axis aberration at locations of large field of view; cause the terms of wavefront aberration function are linked directly to the terms of Zernike polynomials, the aberrated wavefront of the system can be fitted by Zernike polynomials. At the same time, specific terms of Zernike polynomials can be selected to correct wavefront aberration efficiently by evaluating full-field display. Based on the method above, an off-axis optical system with field of view of 3°×3°is designed with two freeform surfaces and one aspheric surface, and certain Zernike polynomials are chosen as the descriptor of freeform surfaces. Keywords:Nodal aberration theory, freeform surface, Zernike polynomials, full-field display.