大气相干长度仪作为测量大气参数的仪器，白天用太阳边缘(夜晚利用恒星)成像在CCD上，利用到达角起伏法测量大气参数。在昼夜交替阶段，肉眼看不到星光而太阳高度角不在可测范围而无法测量。为进行此时间段连续测量，首先要实现低信噪比下星光点目标的检测。在选用合适红外滤波片和高灵敏度CCD的基础上，对采集到的图像进行帧累加和局部增强等处理方法，较好地完成了目标的检测。

Atmospheric coherent length instrument measures parameters with edge of sun in daylight or star at night with fluctuant arrival-angle. But it can't work at the period of day and night alternating when stars can't be observed with eyes and quadrant elevation of sun is unfit for measuring, so image processing with low signal-to-noise ratio is necessary using suitable infrared filter and high-sensitive CCD while observing star in daylight. The experiment proves that little target can be detected well with frame accumulation and local enhancement method.