Astron. Astrophys. Suppl. Ser. 138, 163-176

Acquisition and analysis of adaptive optics imaging polarimetry data

N. Ageorges^1,2 - J.R. Walsh²

Send offprint request: N. Ageorges

1 - National University of Ireland - Galway, Physics Department, Galway, Ireland
e-mail: nancy@physics.ucg.ie
2 - European Southern Observatory, Karl Schwarzschild Strasse 2, D-85748 Garching bei München, Germany
e-mail: jwalsh@eso.org

Received October 5, 1998; accepted June 1, 1999

Abstract:

The process of data taking, reduction and calibration of near-infrared imaging polarimetry data taken with the ESO Adaptive Optics System ADONIS is described. The ADONIS polarimetric facility is provided by a rotating wire grid polarizer. Images were taken at increments of 22.5$^\circ$ of polarizer rotation from 0 to 180$^\circ$, over-sampling the polarization curve but allowing the effects of photometric variations to be assessed. Several strategies to remove the detector signature are described. The instrumental polarization was determined, by observations of stars of negligible polarization, to be 1.7% at J, H and K bands. The lack of availability of unpolarized standard stars in the IR, in particular which are not too bright as to saturate current IR detectors, is highlighted. The process of making polarization maps is described. Experiments at restoring polarimetry data, in order to reach diffraction limited polarization, are outlined, with particular reference to data on the Homunculus reflection nebula around $\eta$ Carinae.

Key words: instrumentation: polarimeters -- instrumentation: adaptive optics -- techniques: image processing -- ISM: $\eta$ car nebulae