From the grating scale monitor to the generalized seeing monitor.

An instrument named the grating scale monitor for measuring the outer scale L0 from the angle-of-arrival (AA) fluctuations of a perturbed wave front was developed a few years ago at Nice University. The AA is detected with a 5-ms time resolution by modulation of the stellar image in a small telescope with a grating. One uses the normalized covariance of AA fluctuations to estimate L0. A new version of this instrument, the generalized seeing monitor (GSM) is described. It consists of four identical modules for measuring the AA at four locations on the wave front. A spatiotemporal analysis of these data leads to the determination of seeing epsilon0, outer scale L0, and the wave-front speed. In addition, isoplanatic angle theta0 is determined from scintillation, making the characterization of turbulence with the GSM almost complete. We describe the instrument and make a detailed analysis of its performance and accuracy. Several site-testing campaigns have been conducted with the GSM: at La Silla (Chile), Oukaïmeden (Morocco), Maidanak (Uzbekistan), and Cerro Pachon and Cerro Paranal (Chile). The main results of these campaigns are presented and discussed.