Intensity profiles before and after subtraction of sky background. The solid lines indicate profiles along the detector y-axis before the subtraction (top line), after the first subtraction (middle), and second subtraction (bottom). The top line is vertically offset by −1000 ADU for visibility. These profiles are averages of the central 20 x-positions. The left half area (y < ~ 200 pix) is the Moon (Earthshine), and the other half is sky. The dashed line is the fit line for the sky region. The corresponding image data after the second subtraction is shown in Fig. 2 (left).