The objective to repeatedly image large areas of sky at seeing-limited resolution led to a unique optical design. The primary mirror is a concave hyperboloid with 4.1 m diameter and about f/1 focal ratio. The mirror has a meniscus shape of 17 cm thickness with a central 1.2 m hole to accommodate the camera at the Cassegrain focus. It was cast from Zerodur by Schott in Germany and subsequently polished and figured by LZOS, Moscow. It is the largest mirror of this shape and of such short focal ratio; polishing it took 2 years, which was longer than anticipated. The mirror is supported by a number of actuators (81 on the back and 24 around the edge), which allow its shape to be controlled by computers.