The main functions of the control system for the very large telescope (VLT) secondary mirror unit are to control the focusing, centering and tilt/chopping movement of the secondary mirror. The most challenging feature of the VLT M2 unit is to tilt the mirror with very high precision and dynamic requirements in chopping and field stabilization mode. During chopping the mirror is tilted up to 10 times per second with a maximum chopping step of 120 arcsec about an arbitrary axis. In the field stabilization mode the mirror has to follow any angular reference signal up to 10 Hz bandwidth with a very high precision. This paper gives an overview on the VLT M2 unit control system with emphasis on the development and realization of chopping control The finally achieved performance and the gained experience are presented.
"Control system of the VLT secondary mirror unit", Proc. SPIE 3351, Telescope Control Systems III, (26 May 1998); doi: 10.1117/12.308792; https://doi.org/10.1117/12.308792