Corning Prepares to Fuse the LSST’s Secondary Mirror (M2) Substrate

Anna Spitz and William Gressler

Corning Incorporated has successfully manufactured and selected all glass material and machined a test piece for a section of the LSST’s secondary mirror (M2) substrate. M2 will be the largest secondary convex mirror ever manufactured and polished. Its extensive size supports the stringent imaging requirements of LSST wide-field science, while permitting occupancy of the camera system within its central hole. Corning technicians are now working through the long, cold, dark, bleak winter in the upper reaches of New York State (Canton) and will continue through next year to produce the M2 annulus shaped substrate (inner hole diameter 1.8 meters and outer diameter 3.5 meters). Corning will complete the substrate in late 2009, and LSST will deliver the Corning substrate to an optical fabricator (not yet selected).

Corning forms its mirror substrates from a series of fusion-sealed Ultra Low Expansion (ULE®) boules. Historically Corning has cut boules into hexagonal shapes for layout and fusing into mirror substrates. But because the LSST secondary has a large inner diameter, technicians will cut the boules into new “petal” shaped segments and arrange them in an annular pattern.

After manufacturing and inspecting the boules for coefficient of thermal expansion (CTE) homogeneity and quality, workers selected the eight required for use in the mirror substrate. During November, they completed machining of one ULE® test boule to demonstrate the first test petal-shaped piece. After they complete machining of all eight boules, they will load the petals into Corning’s 8-meter furnace in a pattern that optimizes CTE homogeneity in the fused monolithic substrate. By late February 2009, the glass will be fusion sealed into a solid piece of glass at a temperature of 1800°C. The plano substrate will be verified for critical zone placement and ground oversize on both surfaces and on the inner and outer circumference. This phase in the process culminates in a 3.5-meter diameter annular plano mirror blank with a thickness of 100 millimeters.

After the mirror substrate is fused workers will again load the plano substrate into the furnace and heat it to 1500°C until it slumps onto, and takes the meniscus shape of, a contoured sag form. Corning will perform the standard ULE® fine anneal at 1000°C after the final heat treatment of the sagging process, which will ensure that it meets minimal residual stresses and final CTE requirements. Technicians will generate the concave back and convex front surface of the substrate as well as the outside and inner diameters on Corning’s 4-meter grinding machine. Corning will provide an aspheric contour ground convex surface to reduce material that the optical polisher will need to remove. Finally Corning will acid etch all substrate surfaces, which provides a pristine surface on which mounts are placed, reduces residual stresses, and increases the overall structural integrity of the substrate. The total schedule for the substrate development will be eighteen months.

LSST chose this design and process after investigating various telescope optical design options. During the design phase, science constraints and telescope packaging drove design criteria: the wide-field necessitates a large convex secondary, while packaging dictates a generous inner diameter to support the camera system. Aspects such as weight limitations, material availability, and cost further defined specifications. Based on the science requirements, design constraints and examples of successful similar mirrors in telescope projects such as SOAR, Discovery Channel Telescope, and VISTA, LSST optics designers settled on the current M2 design. The desire for low expansion glass to meet design specifications limited the choice of available vendors, and Corning provided the winning competitive bid.

After Corning finishes its work, the M2 substrate will move on to the optical fabricator for final polishing to achieve better than 20-nanometer surface accuracy. And after the polishing phase, the fragile secondary mirror will move to Chile for integration into the LSST telescope system on-site. M2 has a long journey ahead—full of delicate and demanding activities. Stay tuned for future updates.