DPR CATG = SCIENCE, DPR TYPE = OBJECT,OzPoz or OBJECT,SimCal

Raw science frame (Medusa mode)

Note:

• All Service Mode data from P80 on have the CCD glow background correctly removed (the bright glow on top right).
• SM science data from 2008-01-18 and later, taken with the SimCal lamp on, have the wavelength scale differentially corrected with the SimCal fibre information .
• As of 2008-04-01, all SimCal data also have information provided (in their product binary table) about barycentric, heliocentric and geocentric RV corrections (in km/s) per fibre. Note that these values have not been applied.

Recipe. The pipeline recipe giscience performs the full reduction of GIRAFFE science frames.

Scheme. The following steps are performed:

• de-bias with a scaled master bias (--bsremove-method=ZMASTER)
• remove background as defined by a master dark scaled by exposure time
• find and extract the fibres, using the localization and width solutions from the flat field
• divide extracted science spectrum by extracted and normalized flat field
• correct science spectra for differences in the fibre transmission (as recorded in the flat field)
• wavelength-calibrate and resample science spectrum, using the dispersion solution
• if SimCal has been used, the wavelength scale is differentially corrected (since 2008-01-18)

The GIRAFFE pipeline run on Service Mode data by QC Garching uses certified and closest-in-time calibration solutions, mostly taken during daytime following the science night. As of 2008-01-18, the recipe takes into account the signal in the simultaneous calibration fibres (OBJECT,SimCal data). Data taken earlier do not have this correction enabled.

Extraction. The fibre signal is extracted by summing up (average extraction) the signal based on the localization solution of the flat field (PLOC file, pro.catg = FF_LOCCENTROID). Note: the signal from the last one or two fibres does usually not fall completely onto the chip. The pipeline suppresses these fibres completely.

The pipeline version 2.5 and later can provide optimum extraction and average extraction. For operational reasons, science data delivered by QC always use the average extraction.

Incomplete last fibre (right); the extracted fibre signal is marked red. This is an extracted view of a raw file, highly compressed in vertical direction in order to emphasize the fibre signal curvature

Flat-fielding. The extracted spectra are divided by the corresponding spectra of the flat field (PFEX file, pro.catg = FF_EXTSPECTRA). The flat-field signal is normalized such that the collapsed spectrum of the object fibre with the highest transmission is set to 1, and all other object fibres are scaled relative to that fibre signal. SKY and SIMCAL fibres are excluded from the statistics and always set to 1. The resulting spectrum has the instrument response curve removed (at the price of now being affected by the lamp continuum which is however very smooth), and to a large extent also the fringing. The figure below demonstrates the effect of flattening: the 20nm scale ondulations are caused by the instrument response, while the ripples on the 1-2 nm scale are due to fringing. Both effects have disappeared after flattening.

Extracted spectrum from a selected fibre (Argus LR773.4, @164). Top: without flat-field correction; bottom: with flat-field correction

Response. There is no response correction done by the pipeline, nor is it foreseen. In Medusa mode, no standard stars are taken, in IFU mode only on request of the user. In Argus mode, spectrophotometric standard stars are measured routinely. These are presently processed like science data. Their products are included in the Service Mode packages and can be used to derive an approximate flux calibration, using the fibres with the STD signal to derive the response curve, while the transmission scaling of the other fibres is provided by the flat field data.

Transmission. The fibre-to-fibre response differences are corrected using informaton from the fibre flats. Their values are stored as column TRANSMISSION in the fibre_setup table (extension 2 of the product files) and can be used to undo the correction.

Sky correction. The sky signal is recorded in the dedicated SKY fibres in the IFU and Argus modes. In Medusa mode, the user is completely free to record the sky signal in any object fibre. Since the pipeline recipe in general does not know exactly which fibres correctly record the sky signal, the sky signal is extracted by the recipe just as for any other fibre, and sky subtraction is left to the user.

Background subtraction. Since October 2007, the recipe subtracts the background as recorded in a master dark frame.

The master dark is constructed from at least 3 input raw darks (one hour exposure time each) and scaled properly for exposure time.

SimCal fibre correction. If the simultaneous calibration fibres have been used, their signal is used to incrementally correct the wavelength scale. The amount of the correction is available as column WLRES in the fibre_setup table (extension 2 of the product files) and can be used to undo the correction.

Since 2008-04-01, all SimCal science data also have information provided (in their product binary table) about barycentric, heliocentric and geocentric RV corrections (in km/s) per fibre. These corrections have not been applied by the pipeline.

Products. The following science products are created by the pipeline:

* coded as HIERARCH.ESO.PRO.CATG in the fits header
** index of the PIPEFILE name, coded as PIPEFILE in the fits header
*** used in the delivered name

The error files (PRO.CATG = SCIENCE_EXTERRORS etc.) contain the calculated standard deviation per pixel of the SCIENCE_EXTSPECTRA file (containing: photon noise, read noise; the flat-field extraction errors is not included here). This error is propagated to the error files of the rebinned spectrum (SCIENCE_RBNERRORS) and, for IFU and Argus, of the recombined spectro-image, SCIENCE_RCERRORS, and the datacube, SCIENCE_CUBE_ERRORS (Argus only).

Data format. All products come as FITS files with the product pixels in the first extension, and a binary table in the second extension. That fibre_setup table is described here.

Quality plots. Since April 2005, we deliver in the Service Mode packages a set of quality control plots.

Medusa. The first QC plot for the Medusa setups has the following elements:

1. a crosscut through the raw frame (row @2048)
2. a closeup of box 1, displaying the central 200 pixels with the fibre PSF and the BIAS level indicated by the red broken line
3. from the extracted and rebinned spectrum file, a vertical collapse indicating the mean signal per fibre
4. a selection of spectra (rebinned for display):

   a) the fibre with the lowest nominal magnitude (read from the ozpoz table), assumed to represent the sky signal

   b) the fibre with the brightest nominal magnitude (again read from the ozpoz table; if none found, fibre 10 is displayed)

   c) S/N for the fibre selected in b), derived by division of the spectrum by the corresponding error in the RBN_ERRORS file (SRBE)

   Both the signal and S/N are plotted under 'slope' with a compressed horizontal scale. The strategy to find the faintest and the brightest fibre assumes the usual magnitudes. If the user has chosen a different convention, the guess may fail.

For spectra above 585 nm, sky emission lines are marked in 4a (red vertical lines), and telluric absorption lines are marked in 4b (blue vertical lines). The emission lines (at UVES resolution) are taken from Hanuschik (2003). The list of telluric lines at the UVES resolution is not yet published.

Three more QC plots for the Medusa modes display all extracted spectra, along with their user-provided target name and magnitude. The figure below shows an example for fibres 1-45. n/a means 'not allocated'.

The Argus and IFU settings also have a set of QC plots. The first one is very similar to the Medusa case, with the faintest object fibre showed under 4a, the integrated signal from all fibres of subslit #6 under 4b, and the S/N for the first fibre from #6. This selection has been made since Argus and IFU spectra are not independent of each other. The guess of the faintest object may fail under the same conditions as described in the Medusa case.

Instead of a preview of all fibres, for Argus and IFU the reconstructed image (RCSPECTRA) is available as preview. It has been obtained by collapsing the spectra into a single number per fibre and display that value on a relative spatial grid. The mapping between fibre index and spatial coordinates has been provided by the pipeline (Fig. 2.7 of the FLAMES manual). Note this is a collapse of the whole spectrum. A narrow-band image in an emission-line can look rather different. Also note that this image is not sky subtracted (but the sky signal is available).