Development of all-sky infrared cloud monitor for the ground based gamma-ray observations

在地面成像大气切伦科夫望远镜观测超高能伽马射线时，云层对观测计划的制定、观测效率和资料质量都有很大影响。因此，本研究的目的是研制一种能够对云层进行连续监测和客观记录的全天红外相机。我们在日本国家天文台研制并投入实际使用的中红外相机的基础上，采用了由光谱响应为8~14μm的非致冷焦平面阵列、凸面反射镜和次镜组成的光学系统组成的光学系统，将视角扩大到整个天空。图像采集由PC上的图形用户界面通过定制的45 m长RS-232电缆进行远程控制。经过适当的定标后，只有在自动偏置模式下对增益进行调整才能获得清晰的图像。晚上，我们可以在显示器上看到肉眼看不到的微弱的云。此外，通过对数字化数据进行一维切片，可以清楚地识别云的位置和相对厚度。在沙漠40度以上高温条件下连续9天的运行测试中，确认系统工作正常，研制了一台全天候红外云量监测仪，并对系统的有效性进行了验证，以期为提高数据质量和有效调度管理提供参考。

For the observations of very high energy gamma rays with the ground based imaging atmospheric Cherenkov telescope, clouds significantly affect making observation schedule, observation efficiency and the quality of the data. So the purpose of this research is to develop and fabricate an all-sky infrared camera to monitor clouds constantly and record them objectively.We used an optical system which consists of a combination of a commercially available infrared camera using an uncooled focal plane array whose spectral response is 8 to 14 μm with a convex mirror reflector and a secondary mirror in order to widen the viewing angle to whole sky, which is based on the mid-infrared camera developed and put practical use by National Astronomical Observatory of Japan. An image acquisition is remotely controlled by GUI on the pc through a custom-made 45 m length RS-232 Cable. After an appropriate calibration, we can obtain clear images only by making an adjustment of gain with auto offset mode. In the night, we can see faint clouds on the monitor disply which are invisible to the naked eye. In addition, by one-dimensional slice of digitized data, the position and the relative thickness of clouds are clearly recognized. We confirmed the system works properly during continuous running test of nine days under extremely hot circumstances over 40 degree in the desert.We developed and fabricate an all-sky infrared cloud monitor, and demonstrated the effectiveness of the system, which is expected to provide the improvement of the data quality and effective scheduling management.