The Sun-Earth Lagrange point L4 is considered as one of the unique places where the solar activity and heliospheric environment can be observed in a continuous and comprehensive manner. The L4 mission affords a clear and wide-angle view of the Sun-Earth line for the study of the Sun-Earth and Sun-Moon connections from he perspective of remote-sensing observations. In-situ measurements of the solar radiation, solar wind, and heliospheric magnetic field are critical components necessary for monitoring and forecasting the radiation environment as it relates to the issue of safe human exploration of the Moon and Mars. A dust detector on the ram side of the spacecraft allows for an unprecedented detection of local dust and its interactions with the heliosphere. The purpose of the present paper is to emphasize the importance of L4 observations as well as to outline a strategy for the planned L4 mission with remote and in-situ payloads onboard a Korean spacecraft. It is expected that the Korean L4 mission can significantly contribute to improving the space weather forecasting capability by enhancing the understanding of heliosphere through comprehensive and coordinated observations of the heliosphere at multi-points with other existing or planned L1 and L5 missions.

We have constructed a wide-field photometric survey system called as the Korea Microlensing Telescope Network (KMTNet) in 2015. It consists of three 1.6 m optical telescopes equipped with mosaic CCD cameras. Four 9k CCDs were installed on the focal plane of each telescope. In this paper, we present the crosstalk analysis of the KMTNet mosaic CCD images. The crosstalk victims caused by bright sources were visible at eight sub-images obtained through different readout ports of each CCD. The crosstalk coefficients were estimated to be several tens of 10-4 in maximum, differing from sub-image to sub-image, and the non-linearity effect certainly appeared at the victims made from saturated sources. We developed software functions to correct the crosstalk effect of the KMTNet CCD images. The software functions showed satisfying results to remove clearly most of the crosstalk victims and have been implemented in the KMTNet image processing pipeline since 2015 September.

We present the characteristics of the 2K CCD camera at the Bohyunsan Optical Astronomy Observatory of the Korea Astronomy Observatory at the time of its development. The purpose of this paper is to support the observers who may need detailed information on the characteristics of the camera and to provide helpful information on the optimization' of a CCD camera for those who try to develop their own camera. The 2K CCD camera was optimized to have a gain of 1.8e−/ADU and a read out noise of 7e− from an experiment using radioactive 55 Fe X-ray source. The charge transfer efficiency was measured as 0.9999976 for serial and 0.9999942 for parallel direction, which means 0.5% charge loss along the serial direction and 1.2% along the parallel direction across the chip. The quantum efficiency of the camera was measured from an experiment using a homogeneous light source consisting of a halogen lamp and an integrating sphere with a monochromator. The resulting quantum efficiency of the camera peaked at the wavelength range 600-700 nm with the value of ＼-0.89 .