Analysis of assembly tolerance compensation in microscope objectives with a free-form element at the aperture stop.

We analyze the feasibility of using refractive free-form phase plates at the aperture stop of microscope objectives as an alternative to active alignment to compensate for assembly tolerances. The method involves the determination of misalignment-induced aberrations at the exit pupil, and transferring them to the aperture stop while taking pupil aberrations into consideration. We demonstrate that despite being able to correct only for field-independent aberrations, this method can restore near-diffraction-limited imaging performance of passively aligned systems with practical tolerances, given that the as-designed system is highly corrected. We confirm the results via numerical simulations for two different commercial objective designs.