Here we report a newly developed method for gravity sag molding of large glass solar reflectors, 1.65 m x 1.65 m square, with either line or point focus, and short focal length. The method is designed for high volume manufacture when incorporated into a production line with separate pre-heating and cooling. The tests reported here have been made in a custom batch furnace, with high power radiative heating to soften the glass for slumping. The mold surface is machined to the required shape as grooves which intersect the glass at cusps, reducing the mold contact area to <1%. Optical metrology of replicas made with the system has been carried out with a novel test using a linear array of coaligned lasers translated in a perpendicular direction across the reflector while the deviation of each beam from perfect focus is measured. Slopes measured over an array of 4000 points show an absolute accuracy of <0.3 mrad rms in sx and sy. The most accurate replicas we have made are from a 2.6 m2 point focus mold, showing slope errors in x and y of 1.0 mrad rms. The slump cycle, starting with rigid flat glass at 500C, uses a 350 kW burst of radiative heating for 200 seconds, followed by radiative and convective cooling.

The first off-axis segment for the Giant Magellan Telescope is being manufactured at the Steward Observatory Mirror Lab. This project includes development of a manufacturing facility and three independent measurements for the seven segments.

We describe the optical fabrication and the active support system of the 6.5 m f/1.25 primary mirror for the first Magellan telescope. Figuring was performed with a 1.2 m stressed lap, which bends under active control to match the local curvature of the optical surface, and small passive tools. The figure was measured with IR and visible interferometers, using refractive null lenses to compensate 810 microns of aspheric departure. After subtraction of Seidel astigmatism and spherical aberration, the finished mirror is accurate to 14 nm rms surface and has an encircled energy of 80% in 0.06 " diameter at 500 nm. The mirror was integrated with its active support system in the laboratory, and support forces were adjusted to optimize the figure. The optimization was performed by singular value decomposition of the influence functions into normal bending modes. Using the first 20 modes and a maximum correction force of 46 N, the surface accuracy is 24 nm rms with 80% of the light in 0.11 " diameter.