DSM-LT: direct slope measurement for large-aperture optical telescope evaluation

Accurate slope measurement in large-aperture optical systems is limited by drift and reference optics. We present DSM-LT-a direct slope measurement technique that employs a 2 × 2 mirror or collimator array with retro-reflected beams. Hardware-based differential detection suppresses common-mode pointing errors without requiring software-based tilt compensation. Wavefronts are reconstructed using a zonal least-squares method, enabling precise localized feature recovery. DSM-LT supports flexible scan patterns and can seamlessly resume after scan interruptions. Simulation results confirm nanoradian-level slope accuracy and robustness against noise. A dual-detector configuration with polarization-separated beams mitigates turbulence effects. DSM-LT provides a compact, resilient, and scalable metrology solution for large-aperture and space-based telescopes.