The DEIMOS flexure compensation system: overview and operational results

Abstract

The DEep Imaging Multi-Object Spectrograph (DEIMOS) was commissioned on Keck II in June 2002. It employs a closed-loop flexure compensation system (FCS) to measure and compensate for image motion resulting from gravitationally-induced flexure of spectrograph elements. The FCS utilizes a set of fiber-fed FCS light sources located at the edges of the instrument focal plane to produce a corresponding set of spots on a pair of FCS CCD detectors located on either side of the science CCD mosaic. (This FCS light follows the same light path through the instrument as the science spectra.) During science exposures, the FCS detectors are read out several times per minute. These FCS images are analyzed in real time to measure any translational motion of the FCS spots and to derive correction signals; those signals drive active optical mechanisms which steer the spots back to their nominal positions, thus stabilizing the images on the FCS CCDs and the science mosaic. This paper describes the commissioning of the DEIMOS FCS system, its continued evolution during its first 18 months of operation on the telescope, and its operational performance over that period. We describe the various challenges encountered while refining the initial FCS prototype (deployed at commissioning) into a fully-operational and highly-reliable system that is now an essential component of the instrument. These challenges include: reducing stray light from FCS light sources to an acceptable level; resolving interactions between FCS acquisition and slit mask alignment; providing robust rejection of cosmic ray events in FCS images; implementing a graphical user interface for FCS control and status.