Real-time atmospheric monitoring for the Cherenkov Telescope Array using a wide-field optical telescope

摘要

The Cherenkov Telescope Array (CTA) is the next generation of ground-basedvery high energy gamma-ray instruments and is planned to be built on two sites(one in each hemisphere) in the coming years, with full array operationforeseen to begin 2020. The goal of performing high precision gamma-ray energymeasurements while maximizing the use of observation time demands detailed andfast information about atmospheric conditions. Besides LIDARs designed tomonitor clouds and aerosol content of the atmosphere in the pointing directionof the CTA telescopes, we propose to use the "FRAM" (F(/Ph)otometric RoboticAtmospheric Monitor) device, which is a small robotic astronomical telescopewith a large field of view and a sensitive CCD camera that together ensureprecise atmospheric characterization over the complete field-of-view of theCTA. FRAM will use stellar photometry to measure atmospheric extinction acrossthe field of view of the CTA without interfering with the observation (unlikelaser-based methods). This allows FRAM to operate with high temporal resolutionand provide both real-time data for on-the-fly scheduling decisions and anoffline database for calibration and selection of scientific data. The fastrobotic mount of the telescope supports quick observation of multiple fieldswhen the array is split and even a check of the conditions in the directions ofthe upcoming observations is possible. The FRAM concept is built uponexperience gained with a similar device operated at the Pierre AugerObservatory. A working prototype of FRAM proposed for CTA is being built inPrague for extensive testing before deployment on site; first results andexperiences with this prototype are presented.