The ESO astronomical site monitor upgrade

Monitoring and prediction of astronomical observing conditions are essential for planning and optimizing observations. For this purpose, ESO, in the 90s, developed the concept of an Astronomical Site Monitor (ASM), as a facility fully integrated in the operations of the VLT observatory([1]). Identical systems were installed at Paranal and La Silla, providing comprehensive local weather information. By now, we had very good reasons for a major upgrade: The need of introducing new features to satisfy the requirements of observing with the Adaptive Optics Facility and to benefit other Adaptive Optics systems. Managing hardware and software obsolescence. Making the system more maintainable and expandable by integrating off-the-shelf hardware solutions. The new ASM integrates: A new Differential Image Motion Monitor (DIMM) paired with a Multi Aperture Scintillation Sensor (MASS) to measure the vertical distribution of turbulence in the high atmosphere and its characteristic velocity. A new SLOpe Detection And Ranging (SLODAR) telescope, for measuring the altitude and intensity of turbulent layers in the low atmosphere. A water vapour radiometer to monitor the water vapour content of the atmosphere. The old weather tower, which is being refurbished with new sensors. The telescopes and the devices integrated are commercial products and we have used as much as possible the control system from the vendors. The existing external interfaces, based on the VLT standards, have been maintained for full backward compatibility. All data produced by the system are directly fed in real time into a relational database. A completely new web-based display replaces the obsolete plots based on HP-UX RTAP. We analyse here the architectural and technological choices and discuss the motivations and trade-offs.