Focal length measurement of large-aperture optical systems based on defocus wavefront sensing

We present a method for measuring the focal length of large-aperture optical systems based on defocus wavefront sensing. A 4D dynamic interferometer acquires Zernike defocus coefficients at different axial positions, from which the focal length is derived via a simplified expression. The method is validated on three optical system models and an off-axis system with a nominal focal length of 50,000 mm. The measured focal length of 49,964 mm agrees well with the design value. The uncertainty analysis yields a total relative uncertainty of 0.184%. The method provides a cost-effective alignment solution requiring no additional large collimators or precision goniometric instruments.