Field rotation
The telescope tracks the stars field, as seen from platform, the star field rotates. This is strongly non linear, from 0.005 arc-sec/s to 2 deg/s and it needs for a counter-rotation system named a derotator. We chose to go for a mirror based derotator to get rid of chromatism effect induced by the glass (Dove prism).
Space consideration design
In order to not loose any optical space on the Nasmyth platform, we chose to look to build a derotator inside the fork of the telescope.
Assembly
The system has been mounted and tested in 2020 without the final mirrors.
