LSST MIRROR PROGRESS

Exciting progress has been made on both the LSST primary/tertiary (M1/M3) and M2 mirror systems. Steward Observatory Mirror Laboratory (SOML) has commenced optical polishing of M1/M3, and a formal request for proposal (RFP) bid package was released for the M2 optical fabrication effort.

M1/M3 Moves into Final Optical Polishing

The M1 and M3 surfaces, which share a single monolithic blank with the M3 positioned inside the M1 annulus ring, are beginning to shine (literally) as they move into final optical polishing. Both surfaces have been generated to rough dimension and located in position via a coarse diamond wheel, removing nearly 5 tons of excess material to allow the steep M3 surface to emerge from within the M1 substrate. Next, the surfaces underwent loose abrasive grinding with finer and finer particle sizes to achieve the initial optical shapes (analogous to sanding a piece of wood with finer grit sandpaper). Now, polishing indicates we have finally reached the last optical processing step. However, this milestone also begins the most challenging aspect of optical fabrication as minute levels of material are removed and the surfaces measured to confirm convergence towards the final optical surface quality.

The polishing process is guided by optical test measurements of the mirror surface taken under the SOML test tower. The photo below left shows preparation for M3 testing (fiducial patches on the mirror relate the test data to the location on the surface). The photo on the right side is a view looking upward to show the bridge structure that supports all the optical test equipment for M3. A similar bridge sits at the very top of the tower supporting the M1 test hardware.

The first optical test data of the M3 surface is shown in the graphic below. This interferogram represents a contour map of the surface, showing the deviation (in height) from the desired final mirror surface. The measurement data is used to determine the next polishing run. The information guides the location and dwell time of the polishing tool to remove small amounts of material, essentially polishing away the high contour areas to make the final smooth optical surface.

Notice the scale of the data is currently in microns of deviation, which certainly is small, but this will eventually become nanometers of deviation as the polishing process is completed. Polishing of M3 and M1 will continue at SOML with final acceptance testing scheduled for late 2013.

M2 RFP Bid Package Released

The Telescope and Site team recently completed a successful M2 Final Design Review in early August to support the release of the RFP for the M2 optical fabrication effort. The baseline scope of work includes polishing the 3.4-meter convex surface and attachment of 78 mirror support pads. Optional scope for the polishing vendor includes the final design and fabrication of the complete M2 Cell Assembly system, consisting of the mirror cell structure, the mirror support system, electronics and sensors, thermal control, and the mirror control system.

The team invested considerable engineering effort to develop a comprehensive baseline system solution to enable a design/build procurement approach. The image below shows a cross section of the M2 Cell Assembly with the integrated M2 (pointing downward). Three rings of actuators comprise the 72 axial supports, which attach to the small pads bonded on the concave backside of M2. These actuators push and pull on the mirror to control the convex mirror shape. Six tangent links around the outer diameter support the transverse loads as a function of zenith angle to minimize stress and deformation in the mirror substrate.

As shown below, the top of the Cell Assembly includes removable panels to enclose the system for thermal control and provides for attachment to the M2 hexapod which aligns the M2 on the telescope mount. The bottom of the Cell Assembly includes aperture rings to define the clear aperture of the M2 and safety stops to prevent any catastrophic damage due to a seismic event. The actuator circular ring geometry enables an efficient steel structure design for the mirror cell. The exploded view of the mirror cell highlights the design features that enable a high natural frequency (to minimize vibration on the telescope), access for maintenance, and clearance holes for cable routing and air flow.

In addition to engineering design and analysis activities, the team performed hardware testing of a prototype actuator. The image to the left shows the baseline actuator design and the prototype axial actuator in a test fixture. Laboratory testing confirmed performance of the component.

Vendor bids are due in early October, with the plan to award the contract to our polishing vendor in early 2013.

Article written by Bill Gressler, LSST Telescope and Site Subsystem Manager