MOS Science data
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.
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)|
Science reduction steps (MOS)
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.