M1/M3 Mirror Update

A mid-March visit to the Steward Observatory Mirror Lab shows the exciting work underway on the LSST M1/M3 mirror with the Large Optical Generator (LOG): diamond generating the front surface curvature of the combined mirror surfaces.

Recall the primary (M1) and tertiary (M3) mirrors are built into a single piece of glass, like two concentric rings, with the outer 8.4-m primary mirror enclosing the 5.0-m tertiary mirror. It will be necessary to remove nearly 5 tons of glass to achieve the approximate shape before polishing can begin. In late February, work began on the M1 surface; attention now has shifted inward to the M3 surface, removing 2 cubic centimeters of glass per second while generating the steeply curved shape to within 5mm of its final thickness. Working from the outer edge to the center, it’s a bit like the needle on a record player spiraling inward, removing a ridge of material in a circular pattern as you work your way around.

The rough generating work shown here is done with a rotating wheel of 20 diamond “pellets”, seen as shiny “teeth” on the underside of the black grinding wheel. A subsequent pass will use a wheel with finer pellets, followed by another pass with resin-bond pellets, each pass smoothing the surface a bit more. Coolant is applied through the blue tubes as the grinding proceeds. The flash image above freezes the wheel rotation to show the diamond pellets and coolant droplets. The safety roller to the right provides real-time feedback to the computer of the glass position.

All this grinding takes place behind the plastic baleen curtain shown at left, while the mirror substrate slowly rotates about once per minute. The generating of the M1/M3 figure should be finished by early May; then on to the grinding and polishing!

Suzanne Jacoby, Dean Ketelsen, and Bill Gressler contributed to this article.