Final acceptance testing of the LSST monolithic primary/tertiary mirror

Abstract

The Large Synoptic Survey Telescope (LSST) is a three-mirror wide-field survey telescope with the primary and tertiary mirrors on one monolithic substrate¹. This substrate is made of Ohara E6 borosilicate glass in a honeycomb sandwich, spin cast at the Steward Observatory Mirror Lab at The University of Arizona². Each surface is aspheric, with the specification in terms of conic constant error, maximum active bending forces and finally a structure function specification on the residual errors³. There are high-order deformation terms, but with no tolerance, any error is considered as a surface error and is included in the structure function. The radii of curvature are very different, requiring two independent test stations, each with instantaneous phase-shifting interferometers with null correctors. The primary null corrector is a standard two-element Offner null lens. The tertiary null corrector is a phase-etched computer-generated hologram (CGH). This paper details the two optical systems and their tolerances, showing that the uncertainty in measuring the figure is a small fraction of the structure function specification. Additional metrology includes the radii of curvature, optical axis locations, and relative surface tilts. The methods for measuring these will also be described along with their tolerances.