Before analyzing the images taken with a Mosaic CCD imager, the images have to reach a state which can be used for further scientic analysis. The transformation of raw images into calibrated images is called data reduction. Transforming HEavely Light into Images (THELI) is a nearly fully automated reduction pipeline software (Erben et al., 2005). This pipeline works on raw images to remove instrumental signatures, mask unwanted signals, and perform photometric and astrometric calibration. Finally THELI constructs a deep co-added mosaic image and a weight map. In this poster, THELI data reduction procedures will be reviewed and the reduction process for raw images of seven X-ray bright groups, extracted from GEMS groups (Osmond & Ponman, 2004) obtained by the Wide Field Imager (WFI) mounted on MPG/ESO telescope at La Silla in March 2006 will be discussed.

ABSTRACT
1. INSTRUMENTS & RAW DATA
2. THELI PIPELINE
3. DATA-PROCESSING
4. PRE-REDUCTION (RUN PROCESSING)
    4.1. Preparation for Data Processing
    4.2. Frame Processing
    4.3. Manual Eyeballing, Mask Creation
    4.4. Creating Weight Frames
5. REDUCTION (SET PROCESSING)
    5.1. Standard Stars Processing
    5.2. Set Distribution
    5.3. Astrometric Calibration
    5.4. Preparation for Coadding
    5.5. Coaddition
6. RESULTS
REFERENCES

• Farnik Nikakhtar(Physics Department, Sharif University of Technology (SUT)/School of Astronomy, Institute for Research in Fundamental Sciences (IPM))