Spiral-phase-contrast imaging based on a linear astigmatism-free three-mirror system for space optics applications

We propose a spiral-phase-contrast imaging system for space optics by integrating a linear astigmatism-free three-mirror system (LAF-TMS) with a spiral phase plate (SPP) in a 4f configuration. The LAF-TMS enables unobscured, off-axis reflective imaging while completely eliminating linear astigmatism across a wide field of view, ensuring robust optical performance suitable for compact space imaging. To enhance phase contrast, we implemented an SPP with a flat central region that maintains a constant phase, generating distinct spiral interference fringes via self-referenced interferometry. These fringes were utilized to reconstruct the 3D topography of a lunar surface-inspired microscale structure, known as the "fairy castle," which models the fragile, porous features typical of the Moon. Simulations confirmed accurate 3D reconstruction with a root mean square error in the sub-micron range. A Monte Carlo-based tolerance analysis further demonstrated that the required alignment precision is achievable with standard optomechanical components, supporting the system's feasibility for in-situ implementation in future space missions.