Mirror Design
Camera | Telescope & Site | Data Management
M1-M3 Monolith
In the LSST optical design, the primary (M1) and tertiary (M3) mirrors form a continuous surface without any vertical discontinuities. Because the two surfaces have different radii of curvature, a slight cusp is formed where the two surfaces meet, as seen in the figure below. This design makes it possible to fabricate both the primary and tertiary mirrors from a single monolithic substrate. We refer to this option as the M1-M3 monolith.
After a feasibility review was held on 23 June 2005, the LSST project team adopted the monolithic approach to fabricating the M1 and M3 surfaces as its baseline. In collaboration with the University of Arizona and Steward Observatory Mirror Lab (SOML) we have begun detailed engineering of the mirror blank and the testing procedures for the M1-M3 monolith. The M1-M3 monolith blank will be formed from Ohara E6 low expansion glass using the spin casting process developed at SOML. This is the same process used to form the mirrors for 8.4-m LBT and 6.5-m Magellan and MMT telescopes. One key difference is in the extra material that must be removed to form the M3 surface. This is because the M1-M3 monolith will be spun for the longer radius of curvature of the M1 surface, leaving excess glass over the M3 surface (red shaded area in the figure below). This extra glass also extends the annealing time required to minimize unwanted stress in the casting by a factor of two.
During the fall of 2005, glass and refractory materials were purchased for the LSST M1-M3 casting. The assembly of the mold for the casting is scheduled to begin in mid-2006 with the actual casting occurring later in the year.
Secondary Mirror
At 3.42 meters in diameter the LSST secondary mirror will be the largest convex mirror ever made. The mirror is aspheric with approximately 17 microns of departure from the best-fit sphere. A sectional view of the secondary reference design is shown below, along with views from behind and in front of the assembly. The design uses a 100 mm thick solid meniscus blank made of a low expansion glass (e.g. ULE or Zerodur) similar to the glasses used by the SOAR and Discovery Chanel telescopes. The mirror is actively supported by 102 axial and 6 tangent actuators. The alignment of the secondary to the M1-M3 monolith is accomplished by the 6 hexapod actuators between the mirror cell and support structure. The large conical baffle is necessary to prevent the direct reflection of star light from the tertiary mirror into the science camera.
All large mirrors are polished with the optical surface facing up. When used in this position the surface print through at the support points is completely compensated for. However, when the mirror is used in an inverted position, with the optical surface facing down, the print through becomes noticeable in the optical figure (see figure at below). Using finite element analysis (FEA), we have modeled this effect for the LSST secondary and have found it to be an acceptable 12.2 nm RMS at zenith and 9 nm at the horizon.
Because weight is a concern for the LSST top end assembly, we have investigated various light-weighting options for the secondary mirror. One option that appears feasible is the structured meniscus shown below. In this design the overall thickness is the same as the solid meniscus, but 1735 lbs of glass is removed leaving a ribbed structure behind a 19-mm face sheet (see figure below). The weight of the structured meniscus is 1500 lbs. The 102 axial supports are in the same geometry as the solid meniscus and are linked by 18 mm thick ribs. Minor ribs support individual pockets to minimize quilting in the optical figure. FEA analysis shows that the six tangential links used in the solid meniscus are insufficient, and 12 are required in order for the structured meniscus to meet the figure requirements.
Additional savings in mass are obtained from the reduced mass of the supporting structures, cell and actuators required to support a lighter mirror. The total mass of the structured meniscus mirror assembly is 5970 lbs (2714 kg), which is 52% of the solid meniscus assembly (11450 lbs, 5205 kg).
