Characterization of an active secondary mirror positioning system for a small telescope

This publication presents the characterization of a modular, active secondary mirror positioning system for a small commercial off-the-shelf Ritchey-Chrétien telescope. Relative displacements between primary and secondary mirror are measured using absolute dimensional metrology, requiring no light from the scientific application. Feedback control is implemented to compensate the measured deviations in three degrees of freedom, by means of a custom-made active mount for the secondary mirror. Tip, tilt, and piston of the secondary are controlled to their calculated ideal position with a closed-loop bandwidth of 2 Hz. The proposed system is characterized under asymmetric thermal influences, which include the small impact of gravitational disturbances, using a Shack-Hartmann wavefront sensor and an artificial star. The conducted experiments successfully demonstrate the applicability of the implemented active compensation approach, decreasing the RMS wavefront error by a factor of more than 7, effectively reducing the introduced optical aberrations.