The infrared cloud monitor for the MAGNUM robotic telescope at Haleakala

We present the most successful infrared cloud monitor for a robotic telescope. The system was originally developed for the MAGNUM 2 m telescope, which has been achieving unmanned, automated monitoring observations of active galactic nuclei at Haleakala on the Hawaiian island of Maui since 2001. Using a thermal imager and two aspherical mirrors, it sees at once almost the whole sky at a wavelength of lambda similar to 10 mm. Its outdoor component is weatherproof and is totally maintenance-free. The images obtained every 1 or 2 minutes are categorized immediately into several ranks of weather conditions, from which our automated observing system not only decides whether to open or close the dome, but also selects what types of observations should be made. The whole-sky data accumulated over 4 years show that 50%-60% of all nights are photometric, and about 75% are observable with respect to cloud conditions at Haleakala. Many copies of this system are now used all over the world, such as those in Mauna Kea in Hawaii, Atacama in Chile, and Okayama and Kiso in Japan.