2 The ADONIS adaptive optics system

2.1 The adaptive optics system

ADONIS is the ADaptive Optics Near Infrared System (see e.g. Beuzit & Hubin 1993 or Beuzit et al. 1997) supported by ESO for common users since December 1994 at the F/8.1 Cassegrain focus of the La Silla 3.6 m telescope. Figure 1 shows the optical layout of the ADONIS adaptive optics (AO) system. A tip-tilt and a 64 element deformable mirror corrects the distortions of the image in real time and a Shack-Hartmann wavefront sensor (WFS) provides the difference signal for the deformable mirror using a bright reference star close to the object. The detector for the Shack Hartmann sensor can be chosen as either an intensified Reticon for bright sources (m_v < 8 mag) or an electron bombarded CCD for fainter sources (8 < m_v < 13 mag, 25 to 200 Hz sampling). Both detectors are sensitive in the visible wavelength region. An off-axis tiltable mirror allows the sky background, in a field of radius $\leq$30'', to be chopped with the on-source image. The output F/45 focus delivers the image to a near-IR detector - either a Rockwell 256² HgCdTe array (SHARP II for $1-2.5~\mu$m, Hofmann et al. 1995) or a LIR HgCdTe 128² anti-blooming CCD (COMIC for $1-5~\mu$m, Marco et al. 1996).

  

Figure 1: Optical layout of the ESO ADONIS adaptive optics system. The polarimeter, as a prefocal instrument, is installed at the entrance window of the dewar (IR focus f/45)

2.2 The camera

The SHARP II camera was selected for the near-IR polarimetric observations. This camera has a fast shutter at the internal cold Lyot stop, allowing integration times as short as 20 msec. The present observations were made with the standard J, H, K filter set and a narrow band 2.15 $\mu$m continuum filter, with a width of 0.017 $\mu$m, and denoted hereafter $K_{\rm c}$.

2.3 The polarizer

 

The polarizer, from Graseby Inc., is a wire grid of 0.25 $\mu$m period, on a CaF₂ subtrate. It is especially designed to work in the spectral range 1 to 9 $\mu$m and has a transmission of 83% perpendicular to the wire grid at 1.5 $\mu$m. It is remotely rotated by the ADOCAM control system to any desired absolute position angle within tolerances of 0.1$^\circ$. This polarizer, as a pre-focal instrument, is inserted into the beam in front of the camera; and is not cooled. However since the polarizer is not oriented perfectly perpendicular to the optical axis there is a small image motion on the detector when rotating the polarizer (see Sect. 4.4 and Fig. 5).