Quality Control and
Data Processing

HC PLOTS
angle offset
polarization offset
instrumental polarization
QC1 database (advanced users): browse | plot

The 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.

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.PMOS.ANGLE.OFFSET	fors2_std_pmos..angle_offset	difference between observed polarization angle and the catalogue value (only for polarized standard stars)	HC		[docuSys coming]
QC.PMOS.ANGLE.OFFSETERR	fors2_std_pmos..err_angle_offset	error of the difference between observed polarization angle and the catalogue value (only for polarized standard stars)	HC		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

Trending

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

Scoring&thresholds Angle Offset

The angle_offset is tightly scored with constant thresholds. The requirement is stability on the calibration timescale.

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.

Algorithm 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. Since polarization parameters are usually tabulated for broad-band filters the pipeline selects the broad-band filter closest to the central wavelength of the grism. Significant offsets may indicate problems with the zeropoint of the retarder plate angle.

The error of the offset in polarization angle is calculated as a difference between the measured and tabulated errors.

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.PMOS.L.OFFSET	fors2_std_pmos..l_offset	difference between observed polarization and the catalogue value	HC		[docuSys coming]
QC.PMOS.L.OFFSETERR	fors2_std_pmos..err_l_offset	error of the difference between observed polarization and the catalogue value	HC		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

Trending

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

Scoring&thresholds Polarization Offset

The relative polarization offset is scored with constant thresholds. The requirement is stability on the calibration timescale.

History

No major events.

Algorithm Polarization 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. Since polarization parameters are usually tabulated for broad-band filters the pipeline selects the broad-band filter closest to the central wavelength of the grism. It is then normalized by dividing through the tabulated value.

The error of the offset in polarization is derived as a difference between the measured and tabulated errors, again normalized by division through the tabulated polarization.

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.PMOS.L.OFFSET	fors2_std_pmos..l_offset	difference between observed polarization and the catalogue value	HC		[docuSys coming]
QC.PMOS.L.OFFSETERR	fors2_std_pmos..err_l_offset	error of the difference between observed polarization and the catalogue value	HC		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

Trending

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

Scoring&thresholds Instrumental Polarization

History

No major events

Algorithm Instrumental Polarization

For unpolarized standard stars (pol_status=N), the pipeline determines the measured value. Since polarization parameters are usually tabulated for broad-band filters the pipeline selects the broad-band filter closest to the central wavelength of the grism. Values significantly different from zero might indicate instrumental polarization. Since the error of the polarization plays an important role here we plot the instruments polarization normalized with its error.