Hermann Boehnhardt

Image motion and flexure compensation of the FORS spectrographs

One of the most critical issues in designing a spectrograph is the motion of opto-mechanical components due to flexure especially when it will be mounted to the Cassegrain focus of a telescope. Image motion on the detector has to be kept small in order not to affect the value of the scientific data. The FORS spectrographs fulfil those requirements by a proper design and by a passive compensation of the instrumental flexure. Image motion of the 2 metric tons instrument could be reduced in this way to a tiny fraction of one pixels size thus not affecting the data gathered with those spectrographs. It is tested and approved at a telescope simulator that all specifications regarding those motions are fully met. A fine tuning flexure compensation is built into the spectrographs design and is tested on its tuning range which allows to adapt the compensation to effects eventually caused by the Cassegrain flange of the telescope.