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The determination of the dispersion relation for MXU data is done as follows: The recipe fors_calib identifies reference lines on MXU 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 FORS2 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. The current version of the pipeline is known to have some problems for slitlets with extreme offsets, that result in arc spectra with few lines.
- average resolution (QC1 database table fors2_mxu_wave, column resol_avg)
This column provides the mean resolution averaged over all spectral lines detected on the complete CCD. It may be misleading if there are small square slitlets, which are used just for centering the mask, but produce of course spectra with a very low resolution.
- mean rms of resolution (QC1 database table fors2_mxu_wave, column resol_meanrms)
This column provides the mean RMS of the resolution per line averaged over the whole wavelength range.
- number of independent identified arc lines (QC1 database table fors2_mxu_wave, column nwave)
This parameter gives the number of independent arc lines that were identified and then used to calculate the above listed parameters.
|top Flatfield Calibration
- number of saturated pixel (QC1 database table fors2_mxu_wave, column n_saturated)
This parameter gives the number of pixel with more than 65000 ADU in the first raw flat frame. The saturation level is 65535 ADU. The counts of the first 10 pixel rows (at the bottom) and the last 10 pixel rows (at the top) are ignored. Before 2012-10-10, the definition of this QC1 parameter was based on the counts of the pipeline processed master spectroscopic flats.
After the mid 2012 intervention to increase the throughput of the
External Calibration Unit (ECU), the ECU lamps were appearing too
bright, causing saturation of flat frames taken particularly with the
150I, 300V and 300I grisms. Eventually, in the summer of 2013 the
exposure times were adjusted and situation stabilized. From June 1,
2013 until November 15, 2013 quality of the spectroscopic calibration
files cannot be guaranteed.