A generalized differential image motion monitor

We present a generalized differential image motion monitor (GDIMM). It is a compact instrument dedicated to the measurement of four parameters of optical turbulence: seeing, isoplanatic angle, coherence time and wavefront coherence outer scale. The GDIMM is based on a small telescope (28 cm diameter) equipped with a three-hole mask at its entrance pupil. The instrument is fully automatic, and performs continuous monitoring of turbulence parameters at the Calern Observatory (France). This paper gives a description of the instrument, data processing and error budget. We also present statistics of three and a half years of monitoring of turbulence parameters above the Calern Observatory.