It is expected that the next generation of large ground based astronomical telescopes will need large fast-steering/tip-tilt mirrors made of ultra-lightweight construction. These fast-steering mirrors are used to continuously correct for atmospheric disturbances and telescope vibrations. An example of this is the European Extremely Large Telescope (E-ELT) M5 lightweight mirror, which is part of the Tip-Tilt/Field-Stabilization Unit. The baseline design for the E-ELT M5 mirror, as presented in the E-ELT Construction Proposal, is a closed-back ULE mirror with a lightweight core using square core cells. Corning Incorporated (Corning) has a long history of manufacturing lightweight mirror blanks using ULE in a closed-back construction, going back to the 1960’s, and includes the Hubble Space Telescope primary mirror, Subaru Telescope secondary and tertiary mirrors, the Magellan I and II tertiary mirrors, and Kepler Space Telescope primary mirror, among many others. A parametric study of 1-meter class lightweight mirror designs showed that Corning’s capability to seal a continuous back sheet to a light-weighted core structure provides superior mirror rigidity, in a near-zero thermal expansion material, relative to other existing technologies in this design space. Corning has investigated the parametric performance of several design characteristics for a 3-meter class lightweight mirror blank for the E-ELT M5. Finite Element Analysis was performed on several design scenarios to obtain weight, areal density, and first Eigen frequency. This paper presents an overview of Corning ULE and lightweight mirror manufacturing capabilities, the parametric performance of design characteristics for 1-meter class and 3-meter class lightweight mirrors, as well as the manufacturing advantages and disadvantages of those characteristics.
It is expected that many of the next generation large ground based telescopes will utilize a segmented design for the primary mirror and, in some cases, the secondary mirror. Corning Incorporated (Corning) presents a process to manufacture segment mirror blanks from Corning ULE titania silicate glass in segment sizes ranging from 1.0 m to 1.8 m flat to flat. This paper will review ULE properties and describe the facilities, equipment, resources, and processes required to produce a few hundred to a few thousand mirror segment blanks for extremely large telescope (ELT) applications.
It is expected that the next generation of large ground based astronomical telescopes will be built using a segmented primary and secondary mirror versus monolithic blanks. Corning has proposed a process to be used in the manufacturing of segment mirror blanks from Corning ULE titania-silica glass in segment sizes ranging from 1.0 to 1.8 meters flat to flat. This paper will describe the facilities, equipment, resources and process required by Corning to produce several hundred to several thousand mirror segment blanks for extremely large telescope mirror blanks.
Fabrication of lightweight mirrors from low expansion glass can be achieved using various core and faceplate lightweighting techniques. Lightweighting can be achieved using abrasive waterjet (AWJ) cutting, CNC machining and a new approach that Corning is developing to produce lightweight cores based on Corning's patented extrusion process. The selection of which lightweighting technology to use is dependent on customer design considerations of the lightweight mirror. These lightweighting techniques are reviewed and discussed, along with advantages and disadvantages of each technique. Corning will also introduce and discuss its new extruded core lightweighting process.
Fabrication of lightweight mirrors from low expansion glasses can be achieved using several techniques. Lightweighting can be obtained through fabrication of fusion-bonded cores, abrasive waterjetting (AWJ) technology and traditional fixed abrasive grinding. Lightweight cores can then be bonded to mirror plates using high temperature fusion, low temperature fusion, and frit bonding processes. Each mirror fabrication approach offers its own advantages and disadvantages in terms of lightweighting, cost and design flexibility. Each of these factors is discussed, along with Corning's proven size capability for each.
ULETM, a zero expansion glass, offers many advantages as mirror blank material due to its thermal and mechanical
properties as well as the flexibility it offers in design and fabrication. Produced by the flame hydrolysis process, this
titanium silicate has high homogeneity of Coefficient of Thermal Expansion (CTE) that can be determined within any
piece of ULE by nondestructive ultrasonic measurements. The ability to fusion-seal the glass while maintaining strength
offers mirror manufacturing design freedom. Coming has produced a 12 inch square plano blank and a 24 inch diameter
meniscus, both fabricated with cooling channels, using existing fusion techniques. The square blank was manufactured by
actually machining grooves into the plates prior to fusing them together. The meniscus production consisted of fusing bars
of ULE between the two outside plates. The meniscus was subsequently thermally tested by NOAO. A 4 meter solid
monolithic meniscus with cooling channels is an extension of this fusion technology.