Design of a high-precision, 0.5  m aperture Cassegrain collimator

We present the realization of a high-precision, 0.5 m aperture size Cassegrain collimator system. The optical design, the optomechanical design, the mirror manufacturing, and the telescope alignment with a performance evaluation are extensively discussed. The optical design of the collimator is based on the Cassegrain telescope design with two aspheric mirrors. An athermalized, high stability optomechanical structure is conceived for the collimator to meet stringent performance requirements. The high-quality mirrors are made of low-expansion Zerodur glass-ceramic and the primary mirror is light-weighted to 63% of its initial weight. The design of a dedicated five-axis flexure mechanism driven by nanopositioner stages to compensate the secondary mirror misalignments is given. Primary and secondary mirrors with aspheric surfaces are manufactured, and their forms are measured by computer-generated holograms with a phase-shifting Fizeau interferometer. The alignment strategy is based on minimizing Fringe Zernike coefficients of wavefront decomposition measured by an autocollimation test setup. The alignment sensitivity and corresponding Fringe Zernike coefficient terms are determined by the ray-tracing software that introduces the intentional misalignments of the secondary mirror. The on-axis alignment of the collimator is performed with the guidance of sensitivity analysis results. The final root-mean-square wavefront error for the collimated beam is measured to be 0.021λ.