A brief history of flux, or the ups and downs of the Na layer density

With the laser guide stars (LGS) becoming a common element of the adaptive optics (AO) systems today and in the future, knowing how much flux you can get at any time and how much variations you can expect is an important input in the design of an AO system. Numerous parameters are affecting the amount of flux available in return for LGSs, in addition to the atmosphere transmission itself: telescope pointing direction, Na column density, time of the year, the year itself, transient effects. The four LGSs of the Adaptive Optics Facility (AOF) on the UT4 telescope at Paranal have been in operation since mid-2016 and the regular logging of the flux detected by the LGS wavefront sensors of the GALACSI (Ground Atmospheric Layer Adaptive Corrector for Spectroscopic Imaging) AO module of the MUSE (Multi-Unit Spectroscopic Explorer) instrument allowed to gather useful statistics of the flux return over almost five years. I will present here the processing of the flux data together with the environmental ones, and the investigation performed to analyse and understand the variations in short, mid, and long periods of time. The statistics of these variations are used to determine the year-by-year and month-by-month flux available for LGS-WFS sensors. These data allow determining the Na density available for LGS generation at different timescales, and the results obtained at Paranal can also be used for Armazones AO systems. These on-sky results also proved to be very valuable for the study of the possibility to split the LGS units on UT4 in the scope of the MAVIS (MCAO Assisted Visible Imager and Spectrograph) instrument.