  In this paper we review THELI (Erben & Schrimer, 2005), an image processing
pipeline developed to reduce multi-pointing optical images taken by mosaic CCD
cameras. This pipeline works on raw images by removing several instrumental
contaminations, implementing photometric calibration and astrometric alignment,
and constructing a deep co-added mosaic image complemented by a weight map. We
demonstrate the procedure of reducing NGC3923 images from raw data to the final
results. We also demonstrate the quality of our data reduction strategy using
mag-count and mag-error in mag plots. Emphasis is mainly placed on photometric
calibration which is of great interest to us due to our scientific case. Based
on the cross-association of the extracted catalogue against a reference
catalogue of stellar magnitudes, zero-point calibration is performed. Our data
reduction strategy and the method employed for cross-correlating large
catalogues is also presented.
