The QC group processes a stream of
science-grade data products for selected VLT instruments. They are created using certified pipelines and certified master calibrations.
Currently these are 1D spectral data products for UVES-ECHELLE, XSHOOTER-SLIT, and GIRAFFE-MEDUSA (MOS modes), as well as IFU datacubes for MUSE and MUSE-DEEP, and HAWKI-IMG data.
Find an overview of the current streams here. (The certification process does not apply for HARPS, FEROS and PIONIER data streams which are currently served through a different, completely automatic process.)
A science raw file is pipeline-processed by the pipeline recipe flames_obs_scired. The bias level and the interorder background are subtracted. Every order of the spectrum is extracted, flat-fielded, de-convolved for fibre cross talk, wavelength calibrated, and corrected for differences in fibre transparency. Finally, the orders are merged.
The UVES-MOS pipeline knows two extraction modes:
Optimum extraction is the default extraction mode. It provides an optimized signal-to-noise ratio. Average (or standard) extraction is an alternative mode which simply averages any signal along the slit and above the sky background level. It may be a good choice in case of failure of the optimum extraction, e.g. at high exposure level.
The pipeline assumes a Gaussian profile for the cross-dispersion flux distribution. This procedure
yields optimum S/N,
automatically discriminates against any non-Gaussian components in the cross-dispersion profile.
This means specifically:
automatic sky subtraction (since the sky background is treated as a pedestal),
automatic cosmic ray rejection (whenever the hit has a non-Gaussian distribution),
automatic sky emission line removal (since these fill up the whole slit and have a rectangular profile).
UVES-MOS offers the possibility to use one fibre for measuring a ThAr spectrum simultaneously with the science observations. The science recipe cross-correlates the simultaneous ThAr spectrum with a template spectrum and determines a velocity shift of the observation with respect to the template. The same calculation is performed with the day-time ThAr calibration.
The cross-correlation is calculated on each order separately. The results are written in the CORVEL_TAB file. The average velocity shift (in km/s) with respect to the template is written in the QC.CCF.POSAVG header keyword; QC.CCF.POSRMS contains the rms of the average and QC.CCF.POSOFF the shift of the day-time ThAr spectrum with respect to the template.
These are the reduction products:
one FIB_SCI_INFO_TAB tfits table containing information about the fibre positions
one MWXB_SCI file per CCD (i.e. two files); this is the merged and wavelength-calibrated extracted spectrum that has been corrected for different throughputs of the fibres; it is a 2D image with wavelength on x-axis and fibre on y-axis
one ERR_MWXB_SCI file per CCD; this is the errorbar file corresponding to MWXB_SCI
one XB_SCI file per CCD; this is a 3D data cube containing the extracted and throughput-corrected spectrum for each order
one MWXB_SCI_RAW file per CCD; similar to MWXB_SCI but without correction for fibre throughput
one ERR_MWXB_SCI_RAW file per CCD; the corresponding errorbar file
one XB_SCI_RAW per CCD; extracted and not throughput-corrected spectra for each order
in case of SimCal data, one CORVEL_TAB table per CCD containing the results of the cross-correlation.