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FORS2 Quality Control:
Polarization Optics

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QC links:
angle offset | polarization offset | instrumental polarization
 
QC PLOTS
  CURRENT HISTORY
angle offset
polarization offset
instrumental polarization
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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.


T
he beams split by the Wollaston are swapped with respect to the instrument design, which causes a rotation of the measured linear polarization angle by 90° degrees. This is compensated by putting the zero point of the half wave retarder plate angle to 45°. Beam swapping has no effect whatsoever on the circular polarisation (for obvious reasons of symmetry).

The parameters described here are derived from spectropolarimetric observations. Since polarization values are usually tabulated per broad-band filter the pipeline selects the broad-band filter closest to the central wavelength of the grism in order to compare to the tabulated values.

top Angle Offset


QC1 parameters

  • angle_offset (QC1 database table fors2_std_pmos, column angle_offset)
    For polarized standard stars (pol_status=Y) the pipeline calculates the difference between the measured angle and the value tabulated in the catalogue. Significant offsets may indicate problems with the zeropoint of the retarder plate angle.
  • angle_offset_error (QC1 database table fors2_std_pmos, column err_angle_offset)
    This provides the error of the offset in polarization angle derived from the measured and tabulated errors.

Trending

Only data with an error in angle offset of less than 10° are trended.

History

When the polarization optics was transfered from FORS1 to FORS2 the zero point of the retarder half wave plate angle was put to 0, which resulted in a measured angle offset of about -88°. This was corrected by July 2, 2009. Between October 21, 2009 and April 7, 2010, an offset angle of about -14° was observed. Since then the offset angle has been consistent with 0.

top Polarization Offset


QC1 parameters

  • l_offset (QC1 database table fors2_std_pmos, column l_offset)
    For polarized standard stars (pol_status=Y) the pipeline calculates the difference between the measured linear polarization and the value tabulated in the catalogue and normalizes it by dividing through the tabulated value.
  • l_offset_error (QC1 database table fors2_std_pmos, column err_l_offset)
    This provides the error of the offset in polarization derived from the measured and tabulated errors, again normalized by division through the tabulated polarization.

Trending

Only data with an error in polarization offset of less than 0.5 are trended.
top Instrumental Polarization


QC1 parameters

  • l_offset (QC1 database table fors2_std_pmos, column l_offset)
    Also for unpolarized standard stars (pol_status=N) the pipeline measures the linear polarization. Significant values may indicate the presence of instrumental polarization.
  • l_offset_error (QC1 database table fors2_std_pmos, column err_l_offset)
    This provides the error of the measured polarization.

Trending

Only data with an error in polarization of less than 0.5 are trended

 
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