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FORS2 Quality Control:
PMOS Wavelength Calibration

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The determination of the dispersion relation for PMOS data is done as follows: The recipe fors_pmos_calib identifies reference lines on PMOS 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. A global distortion tables is ued to identify which spectra belong to the ordinary rays and whioch ones to the extra-ordinary rays. 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.

For each angle of the retarder plate a separate wavelength calibration is observed and processed. For each of these frames a set of QC parameters is determined and stored in a column which has the angle of the retarder plate as identifier. Below this is noted by <retarder_angle>, which can take the values 0_0, 22_5, 45_0, 67_5, 90_0, 112_5, 135_0, 157_5, 180_0, 202_5, 225_0, 247_5, 270_0, 292_5, 315_0, 337_5. Typical angles for circular polarization are 45_0 and 315_0 (RETA4 plate). Typical angles for linear polarization are 0_0, 22_5, 45_0, 67_5 (RETA2 plate).

top Wavelength Calibration

QC1 parameters

  • central wavelength (QC1 database table fors2_pmos_wave, column central_wlen<retarder_angle>)
    This parameter gives the wavelength at the center of field if all slitlets are aligned there. Otherwise this parameter is not calculated. The central wavelength can change slightly due to earthquakes or dismounting of the instrument.
  • central resolution (QC1 database table fors2_pmos_wave, column central_resol<retarder_angle>)
    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 fors2_pmos_wave, column central_resol_meanrms<retarder_angle>)
    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 fors2_pmos_wave, column nwave<retarder_angle>)
    This parameter gives the number of independent arc lines that were identified and then used to calculate the above listed parameters.

top Flatfield Calibration

QC1 parameters

  • number of saturated pixel (QC1 database table fors2_pmos_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.


 
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