Characterization of the sodium layer at Cerro Pachón, and impact on laser guide star performance

Detailed knowledge of the mesospheric sodium layer characteristics is crucial to estimate and optimize the performance of laser guide star (LGS) assisted adaptive optics (AO) systems. In this paper, we present an analysis of two sets of data on the mesospheric sodium layer. The first set comes from a laser experiment that was carried out at Cerro Tololo to monitor the abundance and altitude of the mesospheric sodium in 2001, during six runs covering a period of one year. These data are used to derive the mesospheric sodium column density, the sodium layer thickness and the temporal behaviour of the sodium layer mean altitude. The second set of data was gathered during the first year of the Gemini Multi-Conjugate Adaptive Optics (MCAO) System (GeMS) commissioning and operations. GeMS uses five LGSs to measure and compensate for atmospheric distortions. Analysis of the LGS wavefront sensor (WFS) data provides information about the sodium photon return and the spot elongation seen by the WFS. All these parameters show large variations on a yearly, nightly and hourly basis, affecting the LGS brightness, shape and mean altitude. The sodium photon return varies by a factor of 3-4 over a year, and can change by a factor of 2 over a night. In addition, the comparison of the photon returns obtained in 2001 with those measured a decade later using GeMS shows a significant difference in laser format efficiencies. We find that the temporal power spectrum of the sodium mean altitude follows a linear trend, in good agreement with the results reported by Pfrommer & Hickson.