Precision polarimetry with VLT/SPHERE

Today, we decided to put on arXiv the two long awaited papers about high contrast polarimetry with SPHERE. This work started back in 2014-2015 when Jos was by PhD student and Rob came along for a Diploma internship. 

​The polarimetric imaging mode of VLT/SPHERE/IRDIS I: Description, data reduction and observing strategy.

J. de Boer, M. Langlois, R. G. van Holstein, J. H. Girard, D. Mouillet, A. Vigan, K. Dohlen, F. Snik, C. U. Keller, C. Ginski, D. M. Stam, J. Milli, Z. Wahhaj, M. Kasper, H. M. Schmid, P. Rabou, L. Gluck, E. Hugot, D. Perret, P. Martinez, L. Weber, J. Pragt, J.-F. Sauvage, A. Boccaletti, H. Le Coroller, C. Dominik, T. Henning, E. Lagadec, F. Ménard, M. Turatto, S. Udry, G. Chauvin, M. Feldt, J.-L. Beuzit
http://arxiv.org/abs/1909.13107
Context. Polarimetric imaging is one of the most effective techniques for high-Contrast imaging and characterization of protoplanetary disks, and has the potential to be instrumental in characterizing exoplanets. VLT/SPHERE contains the InfraRed Dual-band Imager and Spectrograph (IRDIS) with a dual-beam polarimetric imaging (DPI) mode, which offers the capability to obtain linear polarization images at high Contrast and resolution. Aims. We aim to provide an overview of IRDIS/DPI and study its optical design to improve observing strategies and data reduction. Methods. For H-band observations of TW Hya, we compare two data reduction methods that correct for instrumental polarization effects in different ways: a minimization of the noise image, and a polarimetric-model-based correction method that we present in Paper II of this study. Results. We use observations of TW Hya to illustrate the data reduction. In the images of the protoplanetary disk around this star we detect variability in the polarized intensity and angle of linear polarization with pointing-dependent instrument configuration. We explain these variations as instrumental polarization effects and correct for these effects using our model-based correction method. Conclusions. IRDIS/DPI has proven to be a very successful and productive high-Contrast polarimetric imaging system. However, the instrument performance depends on the specific instrument configuration. We suggest adjustments to future observing strategies to optimize polarimetric efficiency in field tracking mode by avoiding unfavourable derotator angles. We recommend reducing on-sky data with the pipeline called IRDAP that includes the model-based correction method (described in Paper II) to optimally account for the remaining telescope and instrumental polarization effects and to retrieve the true polarization state of the incident light.

The polarimetric imaging mode of VLT/SPHERE/IRDIS II: Characterization and correction of instrumental polarization effects.
R.G. van Holstein, J.H. Girard, J. de Boer, F. Snik, J. Milli, D.M. Stam, C. Ginski, D. Mouillet, Z. Wahhaj, H.M. Schmid, C.U. Keller, M. Langlois, K. Dohlen, A. Vigan, A. Pohl, M. Carbillet, D. Fantinel, D. Maurel, A. Origné, C. Petit, J. Ramos, F. Rigal, A. Sevin, A. Boccaletti, H. Le Coroller, C. Dominik, T. Henning, E. Lagadec, F. Ménard, M. Turatto, S. Udry, G. Chauvin, M. Feldt, J.-L. Beuzit
http://arxiv.org/abs/1909.13108
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, i.e. the telescope and SPHERE/IRDIS. Methods. We create a detailed Mueller matrix model in the broadband filters Y-, J-, H- and Ks, and calibrate it using measurements with SPHERE's internal light source and observations of two unpolarized stars. We develop a data-reduction method that uses the model to correct for the instrumental polarization effects, and apply 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 H- and Ks-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 unresolved (inner) disks and measuring the polarization of substellar companions. We have incorporated the correction method in a highly-automatic end-to-end data-reduction pipeline called IRDAP which is publicly available at https://irdap.readthedocs.io.
TOI-503: The first known Brown Dwarf-Am star binary from the TESS mission. (arXiv:190

---