Merits of inertial drives for ground-based optical telescopes

To take full advantage of the superb seeing available at the best sites, it is essential to control the attitude of a telescope with the utmost accuracy. Conventionally, this entails supporting the optical elements with as stiff a structure as is feasible and moving the structure with as precise and as smooth a drive system as can be afforded, using a massive pier as the datum for angular measurement and for reacting drive torques. The problems and costs in this approach increase steeply with telescope size, especially if the requirements for minimizing local seeing degradation lead to the telescope being mounted high above the ground or having minimal protection from wind. Ground transmitted vibration may also be a significant problem on some sites. A better approach is to provide the higher frequency angular corrections by supplementary drives which react against angular inertias, analogous to the reaction wheels used for pointing spacecraft. Then, attaining fast correction of errors, such as those induced by wind, does not require particularly stiff structures and drives between the optical assembly and earth. In fact these links with the earth might, with advantage, be made deliberately compliant to isolate the optics from ground vibration. If, in addition, the higher frequency angular errors are sensed relative to an inertial frame, the need for precision gearing is greatly reduced. Computer simulations indicate that the inertial drive technique is quite practical. For very large telescopes, this approach offers better performance at substantially lower overall cost than when conventional drives are used alone.