Optimal defocus for phase diversity in wavefront measurement of rectangular aperture system

Rotational synthetic aperture (RSA) imaging enables super-resolution by rotating a high-aspect-ratio rectangular primary mirror. However, image quality can be compromised by wavefront aberrations that arise from aperture adjustments. The phase diversity (PD) algorithm addresses this by extracting aberrations from pairs of focused and defocused images, with the selection of defocus being a critical factor. For rectangular apertures, the optimal defocus depends on the shape of the aperture. Simulations for aspect ratios m = 1, 2, and 4 show that PD accuracy decreases as the aspect ratio increases, while the optimal defocus value rises from 0.8λ to 2.0λ. Larger aspect ratios show greater resilience to defocus errors. Experimental comparisons confirm that PD achieves superior accuracy when applied at its optimal defocus setting.