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Quality Control and
Data Processing

CAL | HC | refs | QC
Trending & QC1
   ZIMPOL pol std
QC links:
SPHERE: ZIMPOL polarimetric standard star
absolute deviation of total polarimetry in V, R and I
offset in polarimetric angle in V, R and I
Strehl ratio of acquired source
QC1 database (advanced users): browse | plot

The SPHERE calibration plan includes the observations of

  • IRDIS CI/DBI: photometric standard stars and astrometric standard stars
  • IFS: photometric standard stars and astrometric standard stars
  • ZIMPOL-IMG: photometric standard stars and astrometric standard stars
  • ZIMPOL-POL: unpolarized and polarized polarimetric standard stars

ZIMPOL unpolarized and polarized standard stars are acquired once a month in polarimetric mode in three broad band filters, once with closed loop and once with open loop, but both setups without coronograph. A further setup parameters are the neutral density filter.

ZIMPOL standard star observation raw frame.

ZIMPOL standard star observation.

ZIMPOL polarimetric angle and total polarimetry


FITS key QC1 database: table, name definition class* HC_plot** more docu
QC.P.OFFSET sphere_zimpol_stdpol_pol..qc_P_offset offset from catalogue value HC [docuSys coming]
QC.THETA.OFFSET sphere_zimpol_stdpol_pol..qc_THETA_offset offset from catalogue value HC [docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one.


Polarimetric standard stars with high and zero polarimetry are acquired regularly in three broad band filters V, N_R and N_I in open loop and closed loop. The color filter correction is not applied for R and I band. The measured offsets between the measured and the tabulated total polarimetry are monitored.

Scoring&thresholds ZIMPOL polarimetric angle and total polarimetry

The values are not scored.


  • The pipeline recipe to process zimpol poarimetric standard stars was introduced in 2019.
  • Residual instrumemtal polarisation derived from highly polarized standard stars show a larger scatter than that derived from non-polarized standard stars. The cause is not concluded. Several effects are discussed: The set of used highly polarized standard stars is composed of fainter stars than the set of non-polarized standard stars. According to Schmid,H.M. et al.'SPHERE/ZIMPOL high resolution polarimetric imager, I. System overview, PSF parameters, coronography and polarimetry', A&A 619, A9, 2018 (page 26/26) some of the highly polarized standard stars may show polarisation variations.

Algorithm ZIMPOL polarimetric angle and total polarimetry

The sky is subtracted via a numerical aperture and the counts are integrated. From the U and Q Stokes parameters the

total polarimetry P=sqrt(Q^2+U^2)

and the

polarimetric angle (the angle of maximum polarisation) THETA = 0.5 arctan(U/Q)

is calculated. Both values are compared with the catalog values of the star, that P_offset = (P_obs - P_cat ) and THETA_offset = THETA_obs - THETA_cat. For non-polarized standard stars, P_cat is zero and THETA_cat is not defined.

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