2 The instrument

This section is dedicated to a brief description of the instrument NAOS-CONICA. In particular, we emphasize the sub-devices as much as the aspects of design which are important with respect to the static wavefront error estimation by phase diversity. Figure 1 gives an overview of the VLT instrument. NAOS is installed at one of the Nasmyth foci. It picks up an f/15 beam, corrects for atmospheric turbulence and hands on again an f/15 beam providing CONICA with diffraction-limited images. Having passed the Nasmyth focal plane, the beam is led to a first collimating parabola. Then it is reflected successively onto the tip-tilt (TTM) and the deformable mirror (DM). The following dichroic mirror separates the optical train into the imaging path and the wavefront sensing path. NAOS offers five different dichroic beam splitters to adapt for the flux and the spectral characteristics of the guide star. In the imaging path the light is refocused onto the entrance focal plane of CONICA, which is located behind the entrance window in the cold cryostat. Between NAOS and CONICA an atmospheric dispersion compensator (ADC) can be slid in in case of a high zenith angle.

The wavefront sensing path consists of a field selector (Spanoudakis et al. 2000) and two wavefront sensors. They are located between the dichroic mirror and the WFS input focus. For the sake of clarity these components are not shown in Fig. 1. The two wavefront sensors, one in the visible and one in the near infrared spectral range, enhance the sky coverage of possible guide stars. The field selector chooses the guide star in a 2 arcmin field of view and allows differential object tracking, pre-calibrated flexure compensation and counter-chopping. In combination with the deformable mirror it is also able to correct for a certain amount of defocus, as needed when the prisms of the atmospheric dispersion compensator are shifted into the beam. Note that this ability of focus correction offers a possibility to perform PD measurements that we will refer to later on (Sect. 3.2).

The high angular resolution camera CONICA is equipped with an Aladdin array ( $\rm 1K\times1K$) covering the 1-5 $\mu$m spectral region. Splitting the wavelength region into two parts (1 to 2.5 $\mu$m and 2.0 to 5 $\mu$m) allows us to keep the light path achromatic. Therefore the four different pixel scales are realized by seven cameras (Table 1). To each pixel scale a camera is associated with the short wavelengths region (S-camera) and another one with the long wavelengths region (L-camera). The only exception is the camera with the highest magnification (C50S). There is no long wavelength counterpart needed.

A variety of different observing modes is provided by the analyzing optics: chronography, low resolution long slit spectroscopy, imaging spectroscopy by a tunable cold Fabry-Perot, polarimetry by wire-grids or Wollaston prisms, and about 40 broad- and narrow-band filters can be chosen.

Figure 1: Outline of the VLT instrument NAOS-CONICA.

  

Table 1: The required defocus distances for a phase diversity of $2 \pi$ rad (peak to valley) ( $\lambda = 1~ {\mu}$m) are listed corresponding to the f-ratios (pixel scales).