New Technology Telescope (NTT). Astronomer adjusts an active optics actuator on the primary mirror of the NTT at the European Southern Observatory at La Silla, Chile. The NTT's 3.5-metre diameter mirror uses the active optics system to adjust its shape every few minutes. This compensates for the flexure of the mirror when it is moved and for differences in air temperature around the mirror. The NTT's primary mirror has 75 actuators and three fixed points; the pressure exerted by the actuators can be adjusted to make the necessary corrections. The NTT is used for making observations at optical and infrared wavelengths.