Mirror Facets

		Mirror facet size
		Circular/hexagonal mirror facets
		Float glass mirrors
		Mirror mountings
		Mirror coatings
		Mirror alignment

The reviewers concurred with the conclusions of the Mirror Working Group that the optical specifications of the mirrors are best satisfied by hexagonal 60cm mirrors made with a float glass substrate. These mirrors can be produced by the slumping technique in an accurately machined mold which is cost effective in large quantities. The reviewers noted that the specification of substrate quality by reflected flux at the radius of curvature is a good fit for this project: >85% of the reflected flux from a point source at the radius of curvature should fall within a 15mm diameter circle at the point source. This corresponds to a 7.5mm blur on the camera for a point source at $\infty$ and is $\sim 25\%$ of the pixel diameter.

Two methods for mounting mirrors were presented: (1) drill 3 $\sim$15mm holes in a triangular pattern for mounting onto three fine screws with adjusting nuts; (2) a gimbal mount to which the mirror is affixed with a high quality elastomeric adhesive. The motion of the gimbals is effected by high precision stepping motors controlled remotely. Scheme (1) is currently used on the 10m and allows access from the front with a cherry-picker. The reviewers saw merit in both approaches and recommended that more effort be devoted to exploring the mounting designs and developing prototypes.

The reviewers agreed with the proposal of the Mirror Working Group to coat the mirrors with an on-site facility, including aluminization for high reflectance and anodization to protect the surface. They recommended a more precise system be developed for measuring and monitoring reflectance.

The reviewers suggested using the center photomultiplier to check mirror alignment whether using mounting method (1) or (2) described above.