Steward Mirror Lab

The Steward Mirror Lab builds the biggest monolithic telescope mirrors in the world, using a revolutionary honeycomb design and spin-casting furnace. I visited the Mirror Lab on March 8, 2003 on a tour conducted by special arrangement for members of the East Valley Astronomy Club.


The mirror lab is located under the football stadium bleachers on the University of Arizona campus in Tucson, Arizona.

Honeycomb Design

A display of the ceramic honeycomb forms and chunk of borosilicate glass. When melted in the furnace, the glass will flow over and between the forms that fill the mold.

Spin-Casting Furnace

A 6.5-meter diameter mirror in the furnace. Top of furnace has been raised off by crane and stored in bay at upper right. Peak temperature is 1180°C.

Dr. Peter Wehinger, Staff Astronomer at Steward Observatory, acted as our tour guide. He is standing at the edge of the furnace's rotating portion. Rotation produces parabolic shape on upper surface of glass -- the shape required to focus light.

LBT Mirrors

After casting, the ceramic forms are removed, leaving a lightweight honeycomb mirror blank. These are the two 8.4-meter (27-foot) diameter mirrors for the Large Binocular Telescope (LBT) on Arizona's Mt. Graham.

Each LBT mirror weighs 10 tons. The mirror on the left is "face-up," while the mirror on the right is "face-down." They will be the world's largest working in tandem.

The LBT will have the light-gathering capability equivalent to an 11.8-meter diameter mirror (more than any other single telescope), and the resolution of a 22.8-meter diameter mirror (10 times sharper resolution than the Hubble Space Telescope).

The underside of the "face-down" mirror, showing how the fixture used to move the mirror is actually glued to the mirror surface.

Polishing Tool

This is the shaped lap polishing tool under construction. Two computer-controlled laps will change their shape as they move across the face of the mirror. The final surface will be accurate to less than one-millionth of an inch! After installation in the telescope, each mirror will be coated with a thin layer of metal to form the telescope's reflective surface.


These electronically controlled actuators will be positioned behind the mirror to properly support the mirror in different orientations.

All photos taken using Kodak Elite Chrome 400 slide film with Olympus OM-1 35mm camera, scanned with Nikon Coolscan LS-10E or LS-2000.

The Mirror Lab is run by the University of Arizona's Steward Observatory. Tours are by advance arrangement only; call (520) 621-1022 to make an appointment. More information may be found at the Mirror Lab web page.

Revised: September 13, 2007
Copyright © 2003 Joe Orman
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