Volume 572, December 2014
|Number of page(s)||22|
|Published online||20 November 2014|
The Panchromatic High-Resolution Spectroscopic Survey of Local Group Star Clusters
Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität
2 Institute of Astrophysics, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Macul 7820436, Santiago, Chile
3 Gemini Observatory, 670 N. A’ohoku Place, Hilo, HI 96720, USA
4 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
5 Kapteyn Astronomical Institute, 9700 AV Groningen, The Netherlands
Received: 13 May 2014
Accepted: 4 September 2014
Aims. Our dataset contains spectroscopic observations of 29 globular clusters in the Magellanic Clouds and the Milky Way performed with VLT/X-shooter over eight full nights. To derive robust results instrument and pipeline systematics have to be well understood and properly modeled. We aim at a consistent data reduction procedure with an accurate understanding of the measurement accuracy limitations. Here we present detailed data reduction procedures for the VLT/X-shooter UVB and VIS arm. These are not restricted to our particular dataset, but are generally applicable to different kinds of X-shooter data without major limitation on the astronomical object of interest.
Methods. ESO’s X-shooter pipeline (v1.5.0) performs well and reliably for the wavelength calibration and the associated rectification procedure, yet we find several weaknesses in the reduction cascade that are addressed with additional calibration steps, such as bad pixel interpolation, flat fielding, and slit illumination corrections. Furthermore, the instrumental PSF is analytically modeled and used to reconstruct flux losses at slit transit. This also forms the basis for an optimal extraction of point sources out of the two-dimensional pipeline product. Regular observations of spectrophotometric standard stars obtained from the X-shooter archive allow us to detect instrumental variability, which needs to be understood if a reliable absolute flux calibration is desired.
Results. A cascade of additional custom calibration steps is presented that allows for an absolute flux calibration uncertainty of ≲10% under virtually every observational setup, provided that the signal-to-noise ratio is sufficiently high. The optimal extraction increases the signal-to-noise ratio typically by a factor of 1.5, while simultaneously correcting for resulting flux losses. The wavelength calibration is found to be accurate to an uncertainty level of Δλ ≃ 0.02 Å.
Conclusions. We find that most of the X-shooter systematics can be reliably modeled and corrected for. This offers the possibility of comparing observations on different nights and with different telescope pointings and instrumental setups, thereby facilitating a robust statistical analysis of large datasets.
Key words: instrumentation: spectrographs / techniques: spectroscopic / globular clusters: general
Based on observations collected under program 084.D-1061 with X-shooter at the Very Large Telescope of the Paranal Observatory in Chile, operated by the European Southern Observatory (ESO).
© ESO, 2014
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