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 MOS Science data
MOS data reduction | raw files | Reduction steps

MOS data reduction

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. Find an overview of the current streams here.

raw files
MOS data reduction | raw files | Reduction steps

Science Raw Data (MOS)

These types of raw science files exist:
frame DPR TYPE Purpose
SCIENCE OBJECT,OzPoz science exposure (simultaneous calibration fibre not in use)
SCIENCE OBJECT,SimCal science exposure (if simultaneous calibration fibre is used)

Reduction steps
MOS data reduction | raw files | Reduction steps

Science reduction steps (MOS)

Recipe.

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.

Extraction.

The UVES-MOS pipeline knows two extraction modes:

  • optimum (OPT)
  • average (AVG)
    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.

    Optimum extraction.

    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).

    Simultaneous calibration.

    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.

    Products.

    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.

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