## 1 Introduction  
Earth observation and astronomical satellites for acquiring larger-scale, high-resolution images have been developed [1]. The disturbance generated by flywheels for attitude control and other moving components increases the satellite's vibration and degrades the resolution of the captured images. Figure 1 shows that a fast steering mirror (FSM) [2] positioned in the tip-tilt directions can be used to correct the image blur caused by the vibration of the instrument. The mirror needs to have a large diameter to capture large-scale images. It also has to respond rapidly to compensate for high-frequency vibrations. Rapid response of a large mirror is usually difficult to realize because the larger the mirror is, the more vibration modes the FSM has in the low-frequency range. Many FSMs have been developed for corrections of images made by observation satellites and for laser systems for inter-satellite communication. Figure 2 shows a schematic view of an FSM, consisting of a mirror, actuators, displacement sensors, and a guideway mechanism for the tip-tilt motion and axial adjustment of the mirror [3]. Many types of actuator for driving the mirror, such as rotary motors [4], voice coil motors (VCMs) [5], electromagnets [6], and laminated piezoelectric actuators [7], have been tested and used. Electromagnets and laminated piezoelectric actuators generate a large force, but the stroke is smaller than that of a VCM, and non-linearity such as hysteresis and saturation is observed in the input/output characteristics. On the other hand, VCMs can respond rapidly with good linearity and have a larger positioning range than either piezoelectric actuators or electromagnets [5; 6; 7]. Spherical bearings [8], gimbal mechanisms [9], elastic rods [6], and elastic hinges [10] have been designed and tested to realize the tip-tilt motion of FSMs. Spherical bearings have a wide stroke range and a fixed center of rotation, but the friction and wear degrade the positioning accuracy and the response of the FSM. Gimbal mechanisms using flexure pivots [11] are free from friction and wear. However, the structure connecting the mirror and the gimbal ring with some flexure pivots decreases the natural frequencies in the uncontrolled directions. Furthermore, the dynamic characteristics of each axis are different due to the mass and inertia of the gimbal ring, and interference between the axes also increases. A mirror supported by an elastic rod and/or flexure hinges can move without friction and wear, and the guideway mechanism is simple and compact. However, many elastic vibration modes occur in the driving frequency range due to the deformation of the guideway at high frequencies, which destabilizes the feedback system and limits the servo bandwidth. To avoid mode vibration of the mechanical guideways, the use of a magnetic suspension for FSMs has been studied [12]. In this paper, we propose an FSM using a magnetic suspension to guide the mirror in the tip-tilt and axial directions and suppress other low-frequency vibration modes. The proposed magnetic suspension is compact, simple, and free from friction, wear, and maintenance. Furthermore, axial positioning of the mirror for defocus correction of the optical system is also realized. This paper presents the design, fabrication, and control performance of an FSM with an 80 mm diameter mirror. The mirror is supported by a magnetic suspension with two degrees of freedom  
Figure 1: An observation satellite with a fast steering mirror disturbed by reaction wheels. Figure 2: A fast steering mirror (FSM) having a multi-degree-of-freedom (DOF) mechanism. The rotational motion is for jitter compensation. The translational motion is for defocus correction. (DOF) control, and is driven by high response VCMs. The targets for the positioning ranges in the tip-tilt and axial directions are set to be more than \(\pm\)17.4 mrad and \(\pm\)500 mm, respectively. The design target of the servo bandwidth in the tip-tilt directions is set to more than 1000 Hz.