An analysis is made of the application of active optics of various degrees of complexity to large ground-based telescopes, using field stars as reference sources. The performance of active compensation systems is evaluated as a function of the number and size of the active zones, reference star magnitude, turbulence strength, and isoplanatic patch size. The results show that for nighttime observations, the average field star distribution allows real-time compensation not only for quasi-static wavefront errors due to optical misalignment and mirror figure, but also for image motion, dome seeing, and some atmospheric turbulence effects. Such compensation is especially valuable under good seeing conditions, when residual errors become a significant factor. It is suggested that all astronomical telescopes could benefit from the use of compensation systems with even a small number of active zones. In large segmented-mirror telescopes, the segments themselves can be used to compensate for random wavefront errors occurring in the entire optical path. In fixed-primary telescopes, the same function may be performed with an auxiliary deformable mirror.