Manufacture of the combined primary and tertiary mirrors of the Large Synoptic Survey Telescope

The Steward Observatory Mirror Lab is nearing completion of the combined primary and tertiary mirrors of the Large Synoptic Survey Telescope. Fabrication of the combined mirror requires simulation of an active-optics correction that affects both mirror surfaces in a coordinated way. As is common for large mirrors, the specification allows correction of large-scale figure errors by a simulated bending of the substrate with the 156 mirror support actuators. Any bending affects both mirrors, so this active-optics correction is constrained by the requirement of bending the substrate so both mirrors meet their figure specifications simultaneously. The starting point of the simulated correction must be measurements of both mirrors with the substrate in the same shape, i. e. the same state of mechanical and thermal stress. Polishing was carried out using a 1.2 m stressed lap for smoothing and large-scale figuring, and a set of smaller passive rigid-conformal laps on an orbital polisher for deterministic small-scale figuring. The primary mirror is accurate to about 25 nm rms surface after the active-optics correction, while work continues toward completion of the tertiary.