An internal flat field system provides reference flat field images over the spectral range of WFPC2. These are provided by a "calibration channel" optical system mounted outside the main shroud of WFPC2. ... The flat field illumination pattern is fairly uniform for wavelengths beyond about 1600 . Short of 1600 the flat field is distorted due to refractive MgF 2 optics. ... The calibration channel is used primarily to check for internal instrumental stability. ...