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FORS1 Quality Control:
Long Slit Wavelength Calibration

CAL | HC | refs | QC
Trending & QC1
   LSS wavel. cal
Data Management
QC links:
wavelength calibration
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   Click on CURRENT to see the current trending (Health Check).
   Click on HISTORY to see the historical evolution of the trending.

The determination of the dispersion relation for LSS data is done as follows: The recipe fors_calib identifies reference lines on LSS arc lamp exposures, and traces the spectral edges on the associated flat field exposures. With this information the spectral extraction mask to be applied in the scientific data reduction is determined. From the input flat field exposures a normalised flat field frame is also derived. The input arc lamp and flat field exposures are assumed to be obtained quasi-simultaneously, so that they would be described by exactly the same optical distortions. Wavelength calibration is performed row by row and produces a 2-D rebinned science frame. This has the effect of removing the instrumental line curvature and allows the user to extract her/his spectra and have them wavelength calibrated without any additional operation on the frame itself.
The global list of lines currently used by FORS pipeline can be found here.
Due to the high number of possible grism/slit combinations, the Pipeline currently uses a polynomial with a maximum degree of 5 (for 150I without filters) in the dispersion direction, while a 2nd  order polynomial is adopted in the spatial direction. 

top Wavelength Calibration

QC1 parameters

  • central wavelength (QC1 database table fors1_lss_wave, column central_wlen)
    gives the wavelength at the center of the CCD. Due to the curvature of the spectral lines the central wavelength varies along the slit. The central wavelength can change slightly due to earthquakes or dismounting of the instrument.
  • central resolution (QC1 database table fors1_lss_wave, column central_resol)
    The column name of this QC1 parameter is misleading, as it provides the mean resolution averaged over all spectral lines detected on the complete CCD.
  • mean rms of central resolution (QC1 database table fors1_lss_wave, column central_resol_meanrms)
    The column name of this QC1 parameter is again misleading, as it provides the mean RMS of the resolution per line averaged over the whole wavelength range (and not just for the center of the CCD).
  • number of independent identified arc lines (QC1 database table fors1_lss_wave, column nwave)
    This parameter gives the number of independent arc lines that were identified and then used to calculate the above listed parameters.