Honeycomb mirrors for large telescopes

This chapter deals with the design, fabrication, and use of the borosilicateglass honeycomb mirrors which are being produced at the University ofArizona's Steward Observatory Mirror Laboratory. These mirrors are a coretechnology for the whole telescope, and a number of telescopes are nowoperational using these primary mirrors. The mirrors contribute to the telescopedesign because of their light weight, their high stiffness, and their shortthermal time constant. The light weight of the primary mirrors helps to keepthe weight of the entire telescope low and to maximize the structuralperformance. The ability to circulate air through the glass honeycombstructure allows control of local seeing in the telescope environment. Thehoneycomb sandwich is formed by spin casting borosilicate glass into aceramic fiber mold. The Mirror Lab has previously produced three 3.5-mmirrors, three 6.5-m mirrors, and two 8.4-m mirrors which are now operatingsuccessfully in telescopes. Results are highlighted from these telescopes withemphasis on the Large Binocular Telescope with two 8.4 m primaries.Excellent results have been obtained with adaptive secondary mirrorsin combination with the honeycomb primary mirrors. Two additional6.5-m mirrors and two additional 8.4-m mirrors have also been cast andare in various stages of production for other projects including the firstoff-axis segment for the future Giant Magellan Telescope. An additional keytechnology for large telescopes is the ability to fabricate high-precision primaryoptics with short focal lengths in order to keep the telescope structure andenclosure compact. The stressed lap allows efficient polishing of thesefast conic surfaces by actively adjusting its shape as it strokes across themirror.