Manufacture and final tests of the LSST monolithic primary/tertiary mirror

Abstract

The LSST M1/M3 combines an 8.4 m primary mirror and a 5.1 m tertiary mirror on one glass substrate. The combined mirror was completed at the Richard F. Caris Mirror Lab at the University of Arizona in October 2014. Interferometric measurements show that both mirrors have surface accuracy better than 20 nm rms over their clear apertures, in nearsimultaneous tests, and that both mirrors meet their stringent structure function specifications. Acceptance tests showed that the radii of curvature, conic constants, and alignment of the 2 optical axes are within the specified tolerances. The mirror figures are obtained by combining the lab measurements with a model of the telescope's active optics system that uses the 156 support actuators to bend the glass substrate. This correction affects both mirror surfaces simultaneously. We showed that both mirrors have excellent figures and meet their specifications with a single bending of the substrate and correction forces that are well within the allowed magnitude. The interferometers do not resolve some small surface features with high slope errors. We used a new instrument based on deflectometry to measure many of these features with sub-millimeter spatial resolution, and nanometer accuracy for small features, over 12.5 cm apertures. Mirror Lab and LSST staff created synthetic models of both mirrors by combining the interferometric maps and the small highresolution maps, and used these to show the impact of the small features on images is acceptably small.

An insight on optical metrology in manufacturing

“…As an example, figure 21 shows a view of the Large Synoptic Survey Telescope (LSST) test optics looking up into the test tower from the 8.4 m monolithic primary-tertiary mirror under test. The lower bridge contains the tertiary mirror (M3) null test, and the upper bridge contains the primary (M1) test [119].…”

“…In this configuration, a portable deflectometry system measures convex, flat, and concave parts at a very high spatial resolution with the use of an auxiliary lens. This technology is used for various high-quality optics manufacturing projects, including the 8.4 m LSST mirror as shown in figure 27 [119]. In the context of advanced precision optics manufacturing using a sub-aperture computer-controlled figuring tool, correcting mid-to-high spatial frequency errors is one of the most important aspects during the computer-controlled optical surfacing process.…”

Optics technology for large-aperture space telescopes: from fabrication to final acceptance tests

“…The same skills developed and knowledge gained from their projects can be applied to the fabrication of space optics. A recently completed project polishing the combined primary and tertiary mirrors of the Large Synoptic Survey Telescope showcases the tools required to obtain about 20 nm RMS error across an 8.4 m aperture [132]. Three tools were used in the polishing process: a 1.2 m stressed lap [133] with bare pitch or synthetic polishing pads, a rigid-conformal (RC) lap of diameters 35-12 cm based on a non-Newtonian fluid that conforms to the global freeform shape while staying rigid locally [134], and a small pitch lap of diameter 10-5 cm covered with a synthetic polishing pad.…”

Investigation of optical distortion self-compensation ability in the MUltiplexed Survey Telescope

“…Wide-field survey telescopes are widely used to observe tens of thousands of celestial objects, which is crucial for the development of astrophysics [1][2][3][4][5] . To maintain good optical performance and observation quality of a telescope, the primary mirror as a key element in the optical design should have a good surface shape [6][7][8] . In the original design scheme of MUST, the primary mirror with the diameter of 800mm of the central hole is supported by 104 inside diameter load spreaders (IDLS), as is shown in Figure 1(a).…”