SCIENCE DATA: GENERAL

On Paranal, the quick-look pipeline makes an effort to automatically reduce all science data. The reduction is performed using standard calibration solutions from a local calibration database which is refreshed every few months. Generally any pipeline processing on the site is done on a best-effort basis. Its purpose is to offer a quick look to assess data quality etc.

At QC Garching, SCIENCE data taken in Service Mode are pipeline-reduced, using the best available calibration solutions (quality-checked and closest in time, following the calibration cascade). The products of science reduction are also quality-checked. If any irregularity in the reduced data is found, an attempt is made to improve on this. If not possible, information about the problem is made available in the data package. The general policy is to never suppress science data.

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. Data from these two spectrographs taken simultaneously are reduced and packed completely independently, but usually shipped together.

DPR CATG DPR TYPE DPR TECH description FITS headers SCIENCE OBJECT,OzPoz MOS (for Medusa) or IFU (for IFU and Argus) only science and sky observations; no simultaneous calibrations taken science_ozpoz.hdr SCIENCE OBJECT,SimCal MOS (for Medusa) or IFU (for IFU and Argus) simultaneous calibrations taken science_simcal.hdr

SCIENCE PRODUCTS

Overview of science recipe and QC plots

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

Recipe. A science raw file is pipeline-processed by the pipeline recipe giscience. Data are corrected for bias. Optionally a bad pixel map is used to correct for bad pixels (this is not done in QC operations; not required for the new CCD). For the old CCD, the background (composed of dark current and CCD glow) was subtracted using a recent master_dark frame; the new CCD (since 2008-05-26) does not require this. 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 (wave) 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.

Extraction. By default, the GIRAFFE pipeline sums up all pixels within the mask defined in the flat-field localization file. An optimum extraction algorithm is available (but currently not applied in the pipeline scheme used at QC).

Sky. There is no 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. Find more information about the stability of the fibre transmission here.

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. (If you are unhappy with the applied correction, divide these factors into the fibre signals for undoing the correction.)

Fringing. Fringing is removed by the flat-fielding.

Cross-talk and background. With version 2.3, the pipeline can correct for the notorious glow of the old CCD using a close-in-time master_dark.

There is presently no pipeline correction for cross-talk.

Image reconstruction. In the IFU modes (IFU and Argus), a reconstructed image is calculated. 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. With version 2.3, there is also the full 3D datacube delivered for Argus (X,Y,wavelength).

Products. We presently deliver the following types of SCIENCE products:

• SCIENCE_EXTSPECTRA and SCIENCE_EXTERRORS: extracted spectra (with X axis being the fibre index, and Y axis the pixels) and their errors.
• SCIENCE_RBNSPECTRA and SCIENCE_RBNERRORS: rebinned and flat-fielded spectra and their errors (final product).
• For IFU, Argus: the above products, plus SCIENCE_RCSPECTRA, SCIENCE_RCERRORS (reconstructed 2D image and its errors).
• Since June 2007, we also deliver a full 3D data cube for Argus: SCIENCE_CUBE_SPECTRA and SCIENCE_CUBE_ERRORS.
• All product files have a main plane with the pixels, and a binary table extension called fibre_setup table which combines information from the raw file binary tables fibre_table and ozpoz_table. Find more information here.
• Since April 2005, we also deliver for each processed SCIENCE frame a QC plot and preview plots. These are described here.

Since December 2005, all setups (except for three very blue ones) are pipeline-supported. Before that date, most IFU1/2 settings have not been supported.

There is no scattered light correction.

The sky is not subtracted, sky emission lines and sky continuum are imprinted on the science data. The sky signal is available for subtraction by the user from the user-defined SKY fibres.

The following issues with the science reduction are known:

• wavelength calibration covers not always the maximum possible wavelength range (especially at the long end)
• the old CCD ("Bruce", used until 2008-03-13) had a bright glowing area on its upper right edge which does not cancel out upon flat-fielding; it caused an artificial background for fibres 105 - 125 (in the Medusa case) and 250+ (ARGUS, IFU), resp. This glow can be safely removed with a master dark frame. This is implemented in the QC processed data as of P80 (October 1, 2007).
  there is no such issue with the new CCD, no master-dark subtracted.
• three hot columns (at X coordinates 368 to 370, corresponds to fibre #24 for Medusa) are presently not suppressed. There is no meaningful signal in these columns. The extracted signal in this area may be corrupt.
• The pipeline extracts only complete fibres, i.e. the last one or two at the right edge of the chip are not extracted.
• The new CCD "Carreras" has a slightly more variable BIAS than the old one; therefore, it has been decided that the BIAS is removed from all raw frames, incl. SCIENCE data, by averaging the OVERSCAN region into a 1D column, expanding it, and removing it from the raw frames. No master bias is subtracted. However, created master bias frames are archived and delivered.