Polarimetric imaging mode of VLT/SPHERE/IRDIS: II. Characterization and correction of instrumental polarization effects - Archive ouverte HAL
Journal Articles Astronomy and Astrophysics - A&A Year : 2020

Polarimetric imaging mode of VLT/SPHERE/IRDIS: II. Characterization and correction of instrumental polarization effects

R G van Holstein
C U Keller
  • Function : Author
E. Lagadec
  • Function : Author
Jean-Luc Beuzit

Abstract

Context: Circumstellar disks and self-luminous giant exoplanets or companion brown dwarfs can be characterized through direct-imaging polarimetry at near-infrared wavelengths. SPHERE/IRDIS at the Very Large Telescope has the capabilities to perform such measurements, but uncalibrated instrumental polarization effects limit the attainable polarimetric accuracy. Aims: We aim to characterize and correct the instrumental polarization effects of the complete optical system, that is, the telescope and SPHERE/IRDIS. Methods: We created a detailed Mueller matrix model in the broadband filters Y, J, H, and K s and calibrated the model using measurements with SPHERE’s internal light source and observations of two unpolarized stars. We developed a data-reduction method that uses the model to correct for the instrumental polarization effects, and applied it to observations of the circumstellar disk of T Cha. Results. The instrumental polarization is almost exclusively produced by the telescope and SPHERE’s first mirror and varies with telescope altitude angle. The crosstalk primarily originates from the image derotator (K-mirror). At some orientations, the derotator causes severe loss of signal (> 90% loss in the H - and K s -band) and strongly offsets the angle of linear polarization. With our correction method we reach, in all filters, a total polarimetric accuracy of ≲0.1% in the degree of linear polarization and an accuracy of a few degrees in angle of linear polarization. Conclusions: The correction method enables us to accurately measure the polarized intensity and angle of linear polarization of circumstellar disks, and is a vital tool for detecting spatially unresolved (inner) disks and measuring the polarization of substellar companions. We have incorporated the correction method in a highly-automated end-to-end data-reduction pipeline called IRDAP, which we made publicly available online.
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Dates and versions

hal-04712657 , version 1 (27-09-2024)

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R G van Holstein, J H Girard, J. de Boer, F. Snik, J. Milli, et al.. Polarimetric imaging mode of VLT/SPHERE/IRDIS: II. Characterization and correction of instrumental polarization effects. Astronomy and Astrophysics - A&A, 2020, 633, pp.A64. ⟨10.1051/0004-6361/201834996⟩. ⟨hal-04712657⟩
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