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.