Astron. Nachr. / AN 326, No. 6, 432–464 (2005) / DOI 10.1002/asna.200510396 GaBoDS: The Garching-Bonn Deep Survey IV. Methods for the image reduction of multi-chip cameras demonstrated on data from the ESO Wide-Field Imager  T. ERBEN 1 , M. SCHIRMER 1,2 , J. P. DIETRICH 1 , O. CORDES 1 , L. HABERZETTL 1,3 , M. HETTERSCHEIDT 1 , H. HILDEBRANDT 1 , O. SCHMITHUESEN 1,3 , P. SCHNEIDER 1 , P. SIMON 1 , E. DEUL 4 , R. N. HOOK 5 , N. KAISER 6 , M. RADOVICH 7 , C. BENOIST 8 , M. NONINO 9 , L. F. OLSEN 10,8 , I. PRANDONI 11 , R. WICHMANN 12 , S. ZAGGIA 9 , D. BOMANS 3 , R. J. DETTMAR 3 , and J. M. MIRALLES 13,1 1 Institut f¨ ur Astrophysik und Extraterrestrische Forschung (IAEF), Universit¨ at Bonn, Auf dem H¨ ugel 71, 53121 Bonn, Germany 2 Isaac Newton Group of Telescopes, Apartado de correos 321, 38700 Santa Cruz de La Palma, Tenerife, Spain 3 Astronomisches Institut der Ruhr-Universit¨ at-Bochum, Universit¨ atsstr. 150, 44780 Bochum, Germany 4 Leiden Observatory, Postbus 9513, 2300 RA Leiden, The Netherlands 5 Space Telescope European Coordinating Facility, European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei M¨ unchen, Germany 6 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA 7 INAF, Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy 8 Laboratoire Cassiop´ ee, CNRS, Observatoire de la Cˆ ote d’Azur, BP4229, 06304 Nice Cedex 4, France 9 INAF, Osservatorio Astronomico di Trieste, Via G. Tiepolo 11, 34131 Trieste, Italy 10 Copenhagen University Observatory, Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen, Denmark 11 Instituto di Radioastronomia, INAF, Via Gobetti 101, 40129 Bologna, Italy 12 Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany 13 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei M¨ unchen, Germany Received 13 May 2005; accepted 1 June 2005; published online 1 July 2005 Abstract. We present our image processing system for the reduction of optical imaging data from multi-chip cameras. In the framework of the Garching Bonn Deep Survey (GaBoDS; Schirmer et al. 2003) consisting of about 20 square degrees of high-quality data from WFI@MPG/ESO 2.2m, our group developed an imaging pipeline for the homogeneous and efficient processing of this large data set. Having weak gravitational lensing as the main science driver, our algorithms are optimised to produce deep co-added mosaics from individual exposures obtained from empty field observations. However, the modular design of our pipeline allows an easy adaption to different scientific applications. Our system has already been ported to a large variety of optical instruments and its products have been used in various scientific contexts. In this paper we give a thorough description of the algorithms used and a careful evaluation of the accuracies reached. This concerns the removal of the instrumental signature, the astrometric alignment, photometric calibration and the characterisation of final co-added mosaics. In addition we give a more general overview on the image reduction process and comment on observing strategies where they have significant influence on the data quality. Key words: methods: data analysis – techniques: image processing c 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Correspondence to: terben@astro.uni-bonn.de  Based on observations made with ESO Telescopes at the La Silla Observatory 1. Introduction During the last decade, optical Wide-Field Imaging has be- come one of the most important tools in observational astron- omy. The advances in this field are closely linked to the devel- opment of highly sensitive detectors based on charge coupled c 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim