Telescopes need mirrors to reflect and collect light. Before the 1980s, reflective mirrors for telescopes were usually very thick so they can hold their shape to the required precision as it moves across the sky. Their sizes have set a physical limit to how big they can get. Mirrors of this type are usually just around 6m in diameter. Thermal effects such as expansion and contraction also affected the mirror. Big mirrors usually take a long time before reaching thermal equilibrium with the cool night atmosphere.
As astronomy changes, it needed better precision and better pictures of the sky. The 6-meter mirrors are simply not good enough. To improve space imaging, they have constructed very thin and very large mirrors. But instead of applying heavy supports, actuators and controllers are installed to correct the very fragile mirrors and keep it at optimal shape. This system of actuators, image-quality detectors and computer program is collectively called Active Optics. This development has led to the construction of 8-meter mirrors and even larger.
Active optics keeps the large primary mirror at its optimal shape against environmental factors such as gravity for different inclinations, wind, thermal expansion/compression, axis deformation, etc. All factors are corrected within a second or longer to provide the optimum image quality.