In this paper we apply, recently developed, distributed control techniques to the control of an adaptive secondary mirror. The control technique implemented uses the l2 induced norm as a performance criterion. A finite difference model for the mirror is developed using shift operators. We determine the necessary constants from the physical properties of the mirror, from the typical dimensions of such a system, and from our performance requirements for operating the telescope in the near infrared region.The closed loop system is simulated by modelling the deformable mirror in Abaqus, a commercial finite element software, and by implementing the controller in Matlab.
Despite the relatively large number of proposed 'extremely large telescopes' very few of them concentrate on the thermal infrared as their main operating wavelengths. An IR-optimized large telescope located in the Atacama dessert at about 5500m altitude, where many atmospheric windows in the mid-IR open up, would be ideal to study astronomical targets that are either intrinsically red or heavily obscured by dust. A large aperture in the order of 15 - 20m requires adaptive optics correction out to λ⩽20 μm with the least possible thermal emission from the instrument itself. Here we discuss a specialized, integrated AO system that provides diffraction-limited performance in the thermal infrared (at λ⩾2.5 μm). This approach is very different from the AO systems proposed for other 10m+ class telescopes.
We present the basic concept of such an IR-optimized AO system, based on a 2m chopping adaptive secondary. We derive its technical specifications: configuration, bandwidth, and degrees of freedom show its predicted performance for typical seeing in terms of Strehl ratio as a function of limiting guide star magnitude, wavelength and corrected field-of-view. We also briefly address the science that this AO system/telescope would be ideal for.