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

We describe progress on a closed-loop image motion compensation system (IMCS) for the Multi-Object Double Spectrograph (MODS). The IMCS actively compensates for image motion in the focal plane within the instrument caused by temperature fluctuation, mechanism flexure, and large scale structural bending due to gravity. The system utilizes an infrared laser as a reference beam that shares a light path with the science beam and is detected by an infrared reference detector adjacent to the science detector. The reference detector is read out frequently and detects any image motion in the focal plane. The IMCS compensates for this motion during a science exposure by adjusting the tip and tilt angles of the collimator mirror. A working lab prototype meets specifications and is described.