Optical alignment of a linear astigmatism-free imaging system using a Shack-Hartmann wavefront sensor and merit function regression method

We have designed and fabricated a telescope consisting of three freeform surfaces that satisfy the linear astigmatism-free condition for spaceborne remote sensing. After a coarse optical alignment using a coordinate measuring machine, we measured the wavefront at the center field using a Shack-Hartmann sensor for fine-tuning. The measured wavefront error (WFE), decomposed into Zernike polynomials, was used to define a merit function. The nominal design was adjusted to reproduce the measured WFE by changing the values of misalignments. Based on the simulation results, the position and tilt of the mirrors were fine-tuned using shim rings and shim tapes. After compensation, the root-mean-square WFE was reduced from 123 nm to 86 nm. The measured full width at half maximum of the point spread function also decreased from 16 µm to 14 µm.