Developmental Study on Control of Large Telescope System

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The use of thin meniscus mirror of 20-cm thickness is scheduled for the 7.5-m projectg. Such a thin mirror is easily deformed by a self weight.d It is, therefore, intended to control the supporting fprce of the mirror and to control the mirror surface by the use of actuators. Deflection calculations of the mirror due to the self wight were carried out by the finite element method. It was found that about 390 support points are needed and the accuracy of the supporting force must be as high as 1x10^<-4> for the meximum load of 50-kg. It was concluded that the actuator consisting of a ball-screw and a spring is the most reliable device. The ball-srew is rotated by a motor and pushes the spring transforming the vertical motion to a force. The force is measured by a load-cell. The difference of force signal from the calculated value is fed back to the motor to cancel the difference. The test actuator was constructd and tested extensively. Modifying the load cell and several mechanical parts, the error in the axial support force was made as small as (+-) 5 g (max), which was eauivalent to the accuracy of the tuning-fork type load cell itself. Thus, it is concluded that this design of actuator can be used for controling the mirror surface.