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 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.
Corning’s ULE® is an ultra-low expansion glass used for machine tool blocks to astronomical mirrors. Its primary
alternative is a glass ceramic. In many applications, ion beam milling is used for final surface figuring. Ion milling
removes material at an atomic level and is typically a slow, expensive process. Experiments have determined the upper
limits of removal rate for ion beam milling during optical figuring. The goal was to increase the power density of the ion
beam during figuring to achieve higher removal rates with no negative effects on surface properties. Testing shows that
the removal rate on ULE® is about 50% higher than on glass ceramics under the same conditions. With an increase in
material removal rate, both ULE® and the glass ceramic show an increase in surface roughness. Average birefringence
of both materials increases slightly after milling; however the level of birefringence in the glass ceramic is seven times
larger than for ULE®. Therefore using higher ion milling power densities, the surface figuring of ULE® can be
accelerated to produce shorter processing times without adverse effects on surface properties. This can help lower the
cost for manufacture of ULE® optics.
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