Development of Rapid Active Figure Control Systems for Large Telescope Mirrors

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National Astronomical Observatory is planning to build an 8m telescope (Japan National Large Telescope : JNLT) for optical and infrared astronomy on top of Mauna Kea, Hawaii. A thin meniscus primary mirror with an active optics system will be used for JNLT to achieve the best imaging quality at a reasonable cost. Another important innovation to be used for improving the imaging quality of JNLT is the adaptive optics system. The wavefront aberration due to the atmospheric turbulence is measured and quickly compensated for by rapid adaptive optical components.The achievements of the present research in these regards are as follows :(1) An automatic control system in a closed loop to operate the active optics of a 62cm engineering telescope was established.(2) The turn around time of the control loop was shortened by a factor of about two by improving algorithms.(3) A new type damper system using magnetic fluid was developed for JNLT actuators.(4) Micro thermal convection arising from the mirror during night time was precisely measured and analyzed.(5) An image stabilizer, which measures the rapid image motion of a star due to the atmospheric turbulence and compensates for this motion by a servo control, was successfully constructed.(6) Very thin glass mirrors and piezo-electric actuators to constitute an adaptive optics system were developed and characterized.