The principal stages of pretreatment of the images are carried out directly after the observations by the data processing sequences of STScI . The provided images are corrected for the quantum effectiveness of CCDs and bias. The received files are thus in the form of images FITS flatfieldées containing the 4 images corresponding to the 4 CCDs. This file is accompanied by a file of quality which labels the pixels with problem (hot pixels, died pixels...). For more details, one will be able to refer to the handbook of the WFPC2 .