Opening a Window of Discovery on the Dynamic Universe

LSST Mirror Design

Members of the LSST team gather to celebrate the successful casting of the telescope's 27.5-foot-diameter mirror blank in August 2008.

M1M3 Monolith

In the LSST optical design, the 8.4 m primary (M1) and 5.0 m tertiary (M3) mirrors form a continuous surface. Because the two mirrors curve differently, a slight cusp is formed where they meet, as seen in the picture below. Harris Corporation is manufacturing the LSST’s 3.5-meter secondary mirror (M2), which is a solid meniscus design fabricated from ultra-low expansion glass. Harris will also manufacture the mirror’s cell assembly, which consists of the polished secondary mirror, mirror support system, mirror cell electronics and sensors, thermal control system, and the mirror control system. This design makes it possible to fabricate both the primary and tertiary mirrors from a single monolithic substrate, the M1M3 monolith.

A solid substrate of 8.4 m in diameter (26.7 ft) would be too heavy to move as the telescope changes position on the sky, and would take too much time to equalize its temperature with the ambient air from day to night which can cause the images to “shimmer.” An innovative design using a honeycomb structure makes a stiff but lightweight back to hold the mirror surfaces. The mirror mold is built with the honeycomb cores, the glass is layered on top, and as the mirror oven heats and spins, the glass slumps into the mold forming the back of the substrate. Spinning the hot glass creates a roughly parabolic surface that is ground and polished into its final shape.

The Steward Observatory Mirror Lab at the University of Arizona began casting this mirror in the spring of 2008, and final polish was completed in February 2015. On May 19th 2015, the completed LSST primary/tertiary mirror (M1M3) was safely moved from the UA's Richard F. Carris Mirror Lab (formerly SOML) to long-term secure storage at Tucson International Airport. The mirror will be coated with its reflective surface after it is moved to the Summit Facility on Cerro Pachón.

The LSST's primary and tertiary mirrors are created from a single giant piece of glass.Jim Bracken, Randy Lutz, and Phil Muir (L-R) install cores in the LSST monolithic mirror mold.

 

Chunks of Ohara E6 low expansion glass are loaded into the furnace mold. The loading process takes two days to complete and requires 51,900 pounds of glass.

Secondary Mirror M2

At 3.42 m in diameter, the LSST secondary mirror (M2) is the largest convex mirror ever made.

The blank for LSST's secondary mirror is 3.5 meters in diameter.

The mirror was created using a 100 mm-thick, solid blank made of a low expansion glass similar to that used for the Southern Astrophysical Research (SOAR) and Discovery Channel telescopes. The mirror is supported above and facing the M1M3 monolith and the alignment of the three mirror surfaces is maintained using systems of actuators - small motors that can make tiny adjustments to the position of the mirror from several angles. A large conical baffle prevents the direct reflection of star light from the tertiary mirror into the camera.

Because of this configuration, weight is a concern for the M2 mirror assembly and without modifications, the solid glass mirror and assembly would weigh 11,450 lbs (5205 kg). To reduce the weight, 1735 lbs of glass were removed from the back, leaving a ribbed structure behind a 19 mm sheet of glass as the mirror's face (see above). The final weight of the mirror is 1500 lbs (680 kg), while the total weight of the assembly is now 5970 lbs (2714 kg), 52% of its initial weight. The lighter assembly dramatically reduces the demand on the supporting structures, cell and actuators required to support the mirror.

 

Image Credit: 
Top to bottom, left to right: Howard Lester / LSST; V. Krabbendam, LSST; E. Acosta / LSST Corporation; Ray Bertram / Steward Observatory; LSST

Financial support for LSST comes from the National Science Foundation (NSF) through Cooperative Agreement No. 1258333, the Department of Energy (DOE) Office of Science under Contract No. DE-AC02-76SF00515, and private funding raised by the LSST Corporation. The NSF-funded LSST Project Office for construction was established as an operating center under management of the Association of Universities for Research in Astronomy (AURA).  The DOE-funded effort to build the LSST camera is managed by the SLAC National Accelerator Laboratory (SLAC).
The National Science Foundation (NSF) is an independent federal agency created by Congress in 1950 to promote the progress of science. NSF supports basic research and people to create knowledge that transforms the future.   




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