An image motion compensation system for the multi-object double spectrograph

Abstract

We discuss the performance of the Image Motion Compensation System (IMCS) for the Multi-Object Double Spectrograph (MODS). The system performs closed-loop image motion compensation, actively correcting for image motion in the spectrograph's focal plane caused by large scale structural bending due to gravity as well as other effects such as temperature fluctuation and mechanism flexure within the instrument. Not only does the system control instrumental flexure to within the specifications (0.1 pixels on the science CCD, or 1.5 µm), but it also has proven to be an excellent diagnostic tool for assembling and testing the spectrograph. We describe both the final performance of the system as deployed in the spectrograph as well as the instrumental tests made possible by the IMCS.