Astronomers make bias frames to capture the bias level. Bias frames represent noise levels that occur on a CCD detector when placed in total darkness with an integration time of zero. These frames represent the minimum noise generated by the camera electronics as pixel values are read from the CCD array. This bias level can fluctuate because of things that happen every time the CCD is read out or because extraneous signals are added to the bias level. These un-patterned events can originate in power supplies, nearby electronics, motors, and radio-frequency interference from computers and monitors.