Confocal ellipsoidal displaced Gregorian structure for stand-off millimeter-wave imaging

Reflective-based structures are favorable candidates as the optical section of millimeter-wave and terahertz (THz) stand-off imaging systems due to decreasing frequency-dependent losses and aberrations compared to refractive-based counterparts. In this paper, we propose a simple confocal ellipsoidal displaced Gregorian (CEDG) dual-mirror configuration for stand-off imaging systems that exhibits superior performance in terms of mitigating optical blockage and enabling object scanning. The proposed structure is composed of a standard elliptical main mirror, which focuses the emanated rays on the focal plane, and a modified secondary mirror (SM) that reduces the optical blockage effect and improves the imaging performance of the structure. A detailed step-by-step procedure is proposed for constructing the structure, followed by an analytical study and deriving closed-form design equations. By performing ray tracing simulations, it is observed that the optical blockage of the proposed CEDG structure is reduced compared with the standard Gregorian. Also, the resolution and the depth-of-focus (DoF) are obtained at 1.5 cm and 49.7 cm in full-wave simulations, respectively, demonstrating good agreement with the theoretical predictions. Moreover, the scanning performance of the CEDG structure is investigated by utilizing the tilting of the SM and feed point displacement, presenting a maximum field-of-view (FoV) of 40 cm at a distance of 3 m, which is acceptable for modern practical stand-off imaging systems. Finally, the capability of the proposed structure for tuning the stand-off distance by lateral displacement of the feeding source is verified. As a result, the proposed compact, low-cost configuration shows potential for practical mm-wave and THz stand-off imaging systems.