Pretreatment

In this part, we describe the pretreatment of the data. It consists in correcting the rough images from the telescope of the additive component of the camera (dark current, level pedestal of electronics or bias), of the quantum effectiveness of CCDs, withdrawing interference rings for the images taken in the close infra-red (band I). These corrected images are then installed in a data base which makes it possible to handle them easily. Then, the catalogues of the objects present on the images and a subtraction of the bottom of the images are carried out. Finally, the catalogues are compared with a catalogue of reference to determine the absolute astrometry of the objects.