This Field Guide provides a summary of the methods for determining the requirements of an adaptive optics system, the performance of the system, and the requirements for the components of the system. Many of the expressions are in the form of integrals, in which case the authors show the results graphically for a variety of practical values. This second edition has a greatly expanded presentation of adaptive optics control system design and operation. Discussions of control models are accompanied by various recommendations for implementing the algorithms in hardware.
We describe the design and operation of a high-speed adaptive optics system using a robust H controller. The system is also general purpose—it can be used in almost any application with minimal modifications and can be set up and operated by a minimally trained operator. The demonstrated system uses a wavefront sensor camera operating at 955 frames/s, a Xinetics 37-channel deformable mirror, and a dual processor computer to perform computations. The system exhibits control of up to 5 waves of focus, a closed-loop bandwidth of ~50 Hz, with a residual error of /75 rms.
Typical adaptive optics systems require a deformable mirror to provide high spatial frequency wavefront correction and a separate tip-tilt mirror so that the deformable mirror’s dynamic range is not exhausted on low order aberrations. Having two correction devices requires additional optical relays to be incorporated in the system, which in turn translates into more cost, size and complexity. If the two devices were combined into an integrated wavefront corrector (IWC), the cost, size and complexity of an adaptive optics system could be drastically reduced. This paper outlines the design and operation of an electro-ceramic driven tip tilt stage that has been designed specifically for a 37 channel deformable mirror. The tip tilt system can deliver more than 500 μradians of tilt, 20 microns of piston, and has natural frequencies greater than 400 hertz. The tip-tilt stage has a fast response time and the axis of rotation is centered at the optical surface. This prevents translation and wavefront shear associated with typical tip-tilt mirrors for which the axis of rotation is centered behind the surface of the mirror. The 37 channel deformable mirror has a 7mm actuator spacing and is designed with high temperature and low outgassing materials which are compatible with high temperature coatings. The IWC may be retrofitted with Xinetics actuators to operate at cryogenic temperatures. We also describe the use of this device in a closed loop adaptive optics system and outline its benefits.
We report on the results of experiments that demonstrate a robust control system for a general-purpose adaptive optics system and provide robust stability analysis for such a system. Using a commercially available high-speed CCD camera in the Shack-Hartmann wavefront sensor and a 37-actuator Xinetics deformable mirror, we are able to achieve closed-loop performance sufficient for many astronomical, vision science, or laser communications applications. The control system must be robust for the various applications and the entire system must be easily set-up, calibrated, and run by a minimally-trained operator. An H-infinity controller, which optimizes the closed-loop stability of a system, is implemented and analyzed.
Buiding a compact general-purpse adaptive optics system presents new challenges. By “compact” we mean a complete system less than 0.05 m3 in volume that contains all optics and processing electronics. By “general-purpose” we mean a system that can be used in astronomy, in laser communications, and in commercial and military applications with only minimal modifications specific to the application. This necssarily requires a robust control system that can be easily set-up, calibrated, and run by a minimally-trained operator. Knowing that transport or installation of such a system can misalign some components, we built a control system to accommodate those errors in a fast reconstructor reconfiguruing algorithm. A robust H-infinity control is implemented for closed-loop operation. Results of computer simulations and a series of laboratory demonstrations are presented.