WST is an alt-azimuthal, 12.4-m telescope which provides two concentric, simultaneously available fields – a 3.1 sq. degree one (MOS) at a corrected f/3 Cassegrain focus located above the primary mirror, and a 13 arcmin diameter one (IFS) in a Nasmyth configuration. Within the latter, any 3 x 3 arcmin2 sub-field can be selected and propagated through a Coudé relay, down to the gravity-stable f/35 IFS station located in the telescope pier. All optics are within commercially available dimensions. Design volumes for fiber-fed MOS instruments are located on the azimuth floor, for minimal fibre length. The optical design has been optimised to cover the 0.35 – 1.6 µm wavelength range.

With a Cassegrain focus above the fast f/1 primary mirror, the design allows for a compact and stiff alt-azimuthal structure, with evident cost and performance benefits. The size of the dome is driven by the Nasmyth footprint and the height of the pier, which houses the IFS station.

The primary mirror has a total collecting area of 99 m2 and consists of 78 segments. Setting aside their optical prescription, the segments will be identical to those of the ELT, with the same supports and controls