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Quality Control and
Data Processing

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science data
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Science data
Science data reduction | Raw SCIENCE data types | SCIENCE products | Recipe giscience

Science 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.

GIRAFFE science products

For GIRAFFE Medusa1/2 modes, more than 1 million science-grade 1D spectral data products are available covering the entire data history. They are available on the phase3 archive interface.

Raw SCIENCE data types
Science data reduction | Raw SCIENCE data types | SCIENCE products | Recipe giscience

GIRAFFE science data come in two main types: OzPoz (just the OBJECT and SKY fibres illuminated on the sky), and SimCal (with the additional SIMCAL fibres illuminated by the calibration unit). The data types are identified by the DPR TYPE keyword of the FITS headers. Find information about the general GIRAFFE data format and about its CCD here. The information contained in the simultaneous calibration fibres is applied by the pipeline since January 2008.

The GIRAFFE science frames come per slit which can be MEDUSA1, MEDUSA2, IFU1, IFU2, or Argus.

The SCIENCE varieties further split into two different gratings (HR and LR) with a set of central wavelengths (which are listed here).

Note: It is not visible from the DPR TYPE if a FLAMES OB uses UVES and GIRAFFE at the same time, or just GIRAFFE stand-alone.

SCIENCE OBJECT,OzPoz MOS (for Medusa) or IFU (for IFU and Argus) only science and sky observations; no simultaneous calibration fibre used
SCIENCE OBJECT,SimCal MOS (for Medusa) or IFU (for IFU and Argus) simultaneous calibration fibres used

only science and sky observations; no simultaneous calibration fibre used

SCIENCE products
Science data reduction | Raw SCIENCE data types | SCIENCE products | Recipe giscience

Overview of science recipe

Raw frame recipe description products
SCIENCE, OzPoz; all slits giscience debias, CCD glow removal (old CCD), flat-fielding, extraction, wavelength-calibration SCIENCE_RBNSPECTRA, SCIENCE_RBNERRORS (rebinned spectra and their errors);
SCIENCE_EXTSPECTRA, SCIENCE_EXTERRORS (extracted spectra and their errors)
SCIENCE, SimCal same recipe same as OzPoz, plus correction from simultaneous calibration applied as above
SCIENCE, IFU and Argus only same recipe there is in addition the reconstructed image as above, plus SCIENCE_RCSPECTRA, SCIENCE_RCERRORS (2D reconstructed image and error);
SCIENCE_CUBE_SPECTRA and SCIENCE_CUBE_ERRORS (3D reconstructed image and error), for Argus only

The GIRAFFE calibration scheme, including SCIENCE data reduction, is shown here.

Recipe giscience
Science data reduction | Raw SCIENCE data types | SCIENCE products | Recipe giscience

A science raw file is pipeline-processed by the pipeline recipe giscience. Data are corrected for bias. Optionally a bad pixel map can be used to correct for bad pixels. This step is recommended for the old CCD.

The fibres are then extracted using the localization and width solutions from the flat field data. They are wavelength-calibrated using the dispersion-solution from the arc-lamp data. The spectra are then flat-fielded, to correct for overall efficiency, relative (fibre-to-fibre) efficiency, and fringing. For IFU and Argus data, a reconstructed image and a full 3D datacube are calculated.

Bias removal

For the new CCD (since 2008-05-26), the bias is averaged from the overscan region (columns 1-50) into a 1D profile, and subtracted as a 2D expanded function. This method is called PROFILE (--bsremove-method=PROFILE). It has the advantage that the overall BIAS level is always correct, despite the small fluctuations observed to sometimes occur between BIAS calibration files and SCIENCE files. On the other hand, the overscan region has small residual fluctuations which are probably of random nature. To avoid propagating them as dark and bright stripes and bands into the products of the giscience recipe (and actually also the gistandard recipe), there is the method PROFILE+CURVE which applies a 5th order polynomial fit to the 1d profile before applying the result to the science data.

Result of bias removal method PROFILE. The displayed spectrum is an extracted but not yet binned ARGUS standard star product STD_EXTSPECTRA. Each column is an extracted fibre. The artefacts due to the low-amplitude bands in the overscan region are visible as horizontal structures (columns are still in pixel space). The spectrum displayed is extracted from the vertical cut marked by the white solid line.
Result of bias removal method PROFILE+CURVE. This is the same spectrum after applying the bias method PROFILE+CURVE with a 5th order polynomial. There are much less bands visible, most are removed by the smooth polynomial.

Overscan BIAS profiles. Black is the raw BIAS profile (vertical overscan signal, collapsed over the first 50 columns), red is a smoothed version with a filter, green is the 5th order polynomial fit used by the PROFILE+CURVE method.


By default, the GIRAFFE pipeline sums up all pixels within the mask defined in the flat-field localization file (extraction method 'SUM'). An optimum extraction algorithm is also available. The scattered light is more an issue for the IFU modes than for the Medusa data.


There is no automatic sky subtraction provided by the pipeline. The sky fibres (either the dedicated ones for IFU and Argus, or the user-defined ones for Medusa) are processed in exactly the same way as the target fibres. For the ESO-provided spectral products (Medusa), the set of reduced SKY fibres is delivered. The user then has to select the one appropriate for the science target (presumably the closest in space, or a suitable average sky).

Fibre-to-fibre efficiency

Differences of the fibre-to-fibre efficiency are corrected for by the flat-fielding step. The coefficients used for the correction are stored in the product binary table, column TRANSMISSION.


Fringing is removed by the flat-fielding.

Image reconstruction for IFU modes

In the IFU modes (IFU and Argus), a reconstructed image is processed by the pipeline. It is obtained by collapsing the spectra in dispersion direction, and re-arrange the signal according to the X and Y sky coordinates as provided in the fibre table. There is also the full 3D datacube delivered for Argus (X,Y,wavelength).

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