Law data include signals from objects as well as various noises and lack of uniformity of CCD pixels.

Bias is an inclined component, associating with the reading of CCD data. We can obtain the bias component, as we read data under zero exposure time, keeping the shutter closed. The image of this bias component is a bias frame.

Dark is a noise component, occurred by thermal electrons produced in CCD. We can obtain the dark component, as we integrate reading data under appropriate exposure time, keeping the shutter closed. The image of this dark component is a dark frame

If we subtract dark and bias components from law data, we can remove noises and inclined components.

Sensitivity of CCD pixcels is different one by one. In order to correct lack of uniformity of CCD pixels, we take flat (a flat frame) of a uniform source such as a screen illuminated by a white lamp.

If we divide the subtracted frame by the flat frame, we obtain an object frame of the target object.