Polaris non-tracking differential image motion monitors for autonomous atmospheric turbulence monitoring

Abstract

Conventional Differential Image Motion Monitors (DIMMs) use a motorised mount to acquire and track bright stars over the course of a night, increasing system complexity and limiting their suitability for smaller observatories and remote sites. We present a fixed-pointing DIMM installed at the Observatorio del Roque de los Muchachos, La Palma, which observes Polaris without sidereal tracking. The design combines a sufficiently large field of view to contain the circumpolar motion of Polaris with high-cadence short-exposure imaging for measuring atmospheric image motion. A large-format CMOS detector provides an approximately 2.1◦ × 1.6◦ field, while 2ms exposures are acquired within a smaller region of interest that is automatically relocated as Polaris moves across the detector. We describe the instrument design, autonomous acquisition, and image-reduction procedure. Initial results from a ten-night data set demonstrate repeated non-tracking operation under varying atmospheric conditions. Continued observations will provide the wider range of conditions required for a robust long-term comparison with the co-located ING R2D2 DIMM.