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Our dataset contains spectroscopic observations of 29 globular clusters inthe Magellanic Clouds and the Milky Way performed with VLT/X-shooter. Here wepresent detailed data reduction procedures for the VLT/X-shooter UVB and VISarm. These are not restricted to our particular dataset, but are generallyapplicable to different kinds of X-shooter data without major limitation on theastronomical object of interest. ESO's X-shooter pipeline (v1.5.0) performswell and reliably for the wavelength calibration and the associatedrectification procedure, yet we find several weaknesses in the reductioncascade that are addressed with additional calibration steps, such as bad pixelinterpolation, flat fielding, and slit illumination corrections. Furthermore,the instrumental PSF is analytically modeled and used to reconstruct fluxlosses at slit transit and for optimally extracting point sources. Regularobservations of spectrophotometric standard stars allow us to detectinstrumental variability, which needs to be understood if a reliable absoluteflux calibration is desired. A cascade of additional custom calibration stepsis presented that allows for an absolute flux calibration uncertainty of lessthan ten percent under virtually every observational setup provided that thesignal-to-noise ratio is sufficiently high. The optimal extraction increasesthe signal-to-noise ratio typically by a factor of 1.5, while simultaneouslycorrecting for resulting flux losses. The wavelength calibration is found to beaccurate to an uncertainty level of approximately 0.02 Angstrom. We find thatmost of the X-shooter systematics can be reliably modeled and corrected for.This offers the possibility of comparing observations on different nights andwith different telescope pointings and instrumental setups, therebyfacilitating a robust statistical analysis of large datasets.

astronomy and astrophysics

The Panchromatic High-Resolution Spectroscopic Survey of Local Group  Star Clusters - I. General Data Reduction Procedures for the VLT/X-shooter  UVB and VIS arm