Development of an infrared all-sky camera system for cloud monitoring

Automatic survey observations require a high-resolution cloud monitor to navigate a telescope system to a clear sky. A wide-field thermal camera is a powerful tool for monitoring cloud distribution regardless of scattered lights from the Moon and nearby cities. We developed a compact mid-infrared all-sky monitor using a waterproof camera module with a Germanium lens and an 80 x 80-pixel silicon-bolometer array for wavelengths from 8 to 12 mu m and a magnifying mirror with a diameter of 100 mm. The magnifier has a rotationally symmetrical shape with a round-shaped cross-section, which produces a radially reversed all-sky image without a block of the view by the camera body. The camera system is equipped with image processing software to transform coordinates and linearize the response of the sensor. In long-term test observations at the Kiso Observatory, we confirmed that the system obtained frames of the sky every 60 seconds with a sufficient sensitivity to discriminate between clear and obscured areas.