of Zerodur is used to form a Ritchey Chretien f/11 Cassegrain system with a 15 arcmin field of view. An additional flat fold mirror allows the use of any one of two Nasmyth platforms or two folded Cassegrain stations, each with 5 arcmin fields of view. The telescope sometimes operates in a wide-field prime focus configuration, in which case the secondary is removed and a three element field-correcting lens inserted, which provides an effective f/2.8 focus with a 60 arcmin field of view (40 arcmin unvignetted). Changing between the Cassegrain and Nasmyth foci takes a matter of seconds and may be done during the night; switching to and from prime focus requires replacing the secondary mirror with a prime focus assembly during daytime (the two are mounted back-to-back) which takes around 30 minutes. A Coudé focus was planned as a later addition, to feed an optical interferometer with another telescope, but this was never built. A chopping f/35 secondary mirror was planned for infrared observations, but was placed on hold by the cost-saving re-design and never implemented. Mount The optical system weighs and is manoeuvred on an alt-azimuth mount, with a total moving mass of (plus instruments). The BTA-6 and Multi Mirror Telescope had demonstrated during the 1970s the significant weight (and therefore cost) savings which could be achieved by the alt-azimuth design compared to the traditional equatorial mount for large telescopes. However, the alt-azimuth design requires continuous computer control, compensation for field rotation at each focus, and results in a 0.2 degree radius blind spot at zenith where the drive motors cannot keep up with sidereal motion (the drives have a maximum speed of one degree per second in each axis). The mount is so smooth and finely balanced that before the drive motors were installed it was possible to move the then assembly by hand. During closed loop guiding, the mount is capable of an absolute pointing accuracy of 0.03 arcseconds. Dome The telescope is housed in an onion-shaped steel dome with an internal diameter of , manufactured by Brittain Steel. The telescope mount is located on a cylindrical concrete pier so that the centre of rotation is above ground level, which lifts the telescope above ground-layer air turbulence for better seeing. A conventional up-down 6m-wide shutter with wind-blind, several large vents with extractor fans for thermal control, and a capacity crane (used for moving the primary mirror