In the framework of the ``Laser Guide Star for 8-m Class Telescopes" Training and Mobility of Researchers network funded by the European Union, an IDL-based software package has been developed to simulate generic adaptive optics (AO) systems. The structure of the software is modular. Each elementary physical process such as turbulence in atmospheric layers, propagation of light from source to observing telescope and through the turbulent layers, the wavefront sensor, is modeled in a specific module. The resulting software, called Code for Adaptive Optics Systems (CAOS), is composed of a global graphical user interface (GUI), the CAOS Application Builder (Fini et al. 2001), and a set of specific modules--the CAOS Simulation Package. A list of modules and brief descriptions are presented in Section 2. An example of an application to the Large Binocular Telescope (LBT) interferometer AO system is presented in Section 3. References to more information about CAOS are given in Section 4.
Table 1 shows a complete list, together with a very brief description, of the modules of CAOS Simulation Package 3.0.
Using the CAOS Application Builder, a simulation can be built by connecting together the required occurrences of the desired modules, represented by the boxes of Figure 1. The only constraints are those imposed by input/output types. Each module comes with an individual GUI in order to set its own physical and numerical parameters, during the design step or independently at a later time. The whole structure of a simulation can be saved as a ``project" that can be restored for later modifications and/or parameters upgrading. The IDL code, corresponding to the designed simulation, is written down during the saving of a project, and it can be modified ``by hand" in order to be completed with additional tasks not supported by the CAOS package.
Let's assume that we would like to simulate high-angular-resolution observations in the red and near-infrared wavelength bands1, with the LBT interferometer2 and with AO correction. Figure 1 (left) shows the whole project designed for such a purpose, with a natural guide star (NGS) of 10 magnitude for the AO sensing, either on-axis or 1 off-axis with respect to the astronomical object. The turbulent atmosphere is modeled with two layers: a ground layer evolving along the baseline and weighted with 30% of the total turbulent energy, and an upper layer (at 10km altitude) evolving orthogonally to the baseline and weighted with 70% of the total turbulent energy. For both layers the wind speed is 5m/s. The total Fried parameter is 20cm (at 500nm), and the wavefront outer-scale is 40m. Each pupil of the LBT interferometer has its own AO system made of a tip-tilt (TT) correction loop, and a high-orders (HO) correction loop. The HO loop is made of: a 3434 Shack-Hartmann lenslet array with 88 pixels/sub-aperture and 0.15/pixel for the sensing, a modal rejection of the Zernike modes over number 231 (20 radial order) during the wavefront reconstruction, and a 3535 actuators deformable mirror (that corresponds to a projected inter-actuator distance of 23.5cm on the primary mirror) for the correction. The TT loop contains a quad-cell detector (with 0.25/cell). Both sensings are performed in R, the light from the NGS being split 95% for the HO loop and 5% for the TT loop, assuming an overall efficiency of 60% and a read-out noise of 3 e. The time-filtering acts in each loop as a pure integrator, and the differential piston is supposed to be perfectly corrected. The scientific CCD - on which the point-spread function (PSF) corresponding to the astronomical object is formed - make 128128 pixels images3. The total temporal history of each simulation run - one per value (0, 60, and 120) of the parallactic angle - is 2.075s (corresponding to 415 iterations of 5ms each), but the resulting interferometric PSFs (one per band (four) and per off-axis (two) considered) are integrated over the last 2s for sake of AO stability. For each of the simulation run, a different realization of the turbulent atmosphere was considered (since each parallactic angle corresponds to a different period of the observing run). Figure 1 (right) shows two of the 342 obtained PSFs, while Table 2 synthesizes the quality of all the 24 AO-corrected interferometric PSFs obtained in terms of Strehl ratio.
For more information and references on the CAOS Simulation Package and the CAOS Application Builder, see http://www.arcetri.astro.it/caos. See also Correia et al. (2001) for a description of the CAOS-compatible simulation package AIRY (Astronomical Image Restoration in interferometrY).
Fini, L., Carbillet, M., Riccardi, A. 2001, this volume, 253
Correia, S., Carbillet, M., Fini, L., et al. 2001, this volume, 404