Astronomy and Astrophysics – Astrophysics – Instrumentation and Methods for Astrophysics
Scientific paper
2010-10-22
Astronomy and Astrophysics
Astrophysics
Instrumentation and Methods for Astrophysics
16 pages, 15 figures, 5 tables, accepted by A&A on 2010 October 11
Scientific paper
We report on the results of calibrating and simulating the instrumental polarization properties of the ESO VLT adaptive optics camera system NAOS/CONICA (NACO) in the Ks-band. We use the Stokes/Mueller formalism for metallic reflections to describe the instrumental polarization. The model is compared to standard-star observations and time-resolved observations of bright sources in the Galactic center. We find the instrumental polarization to be highly dependent on the pointing position of the telescope and about 4% at maximum. We report a polarization angle offset of 13.28{\deg} due to a position angle offset of the half-wave plate that affects the calibration of NACO data taken before autumn 2009. With the new model of the instrumental polarization of NACO it is possible to measure the polarization with an accuracy of 1% in polarization degree. The uncertainty of the polarization angle is < 5{\deg} for polarization degrees > 4%. For highly sampled polarimetric time series we find that the improved understanding of the polarization properties gives results that are fully consistent with the previously used method to derive the polarization. The small difference between the derived and the previously employed polarization calibration is well within the statistical uncertainties of the measurements, and for Sgr A* they do not affect the results from our relativistic modeling of the accretion process.
Araujo Carolina
Bremer Malcolm
Buchholz Rainer M.
Eckart Andreas
Karas Vladimir
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