Ultra-Low Expansion (ULE®) glass has been and continues to be a significant material for astronomical applications.
With a nominal composition of 7 wt. %TiO2 in SiO2, Corning Code 7972 ULE® has a mean room temperature
coefficient of thermal expansion (CTE) of 0 ± 30 ppb/°C with a typical CTE range of less than 15 ppb/°C, properties vital
to the manufacture of high resolution optics requiring extreme thermal stability. Combined with lightweighting
techniques developed at Corning during the past 30 years, ULE® has been successfully employed for numerous
monolithic and lightweight mirror applications including the 2.4 meter Hubble Space Telescope lightweight primary
mirror, the Airborne Laser (ABL) primary mirrors, and most recently the Discovery Channel Telescope 4 meter mirror
blank. ULE® maintains its strong candidacy for future ELT applications.
Recent challenges in mirror surface specifications and the development of alternative material choices calls for a
comparison with ULE®. The objective of this article is to review ULE® properties and manufacturing capabilities, and to
compare relevant material properties to those of alternative material options, thus allowing designers to properly execute
material selection. Finally, recent development efforts directed toward improving ULE® will be discussed.
Corning manufactures several optical materials that can be used as reflective and transmissive optics for telescope optical systems. Corning can manufacture these materials in a large range of sizes and configurations. This paper discusses Corning's portfolio of optical materials and their properties, along with Corning's manufacturing capabilities using these materials. Specific examples of optical blanks that Corning has supplied will be discussed.
Corning Incorporated has had success with many of the large optics for different programs. Corning's ability to fuse
ULE® monolithic blanks from smaller portions allows for a wide variety of shapes and configurations. Corning's latest
success has come with the Discovery Channel Telescope (DCT) 4.3 meter ULE® blank, produced for Lowell
Observatory. This paper will document the process for the fabrication of the blank from inception to completion and
shipment of the blank to the customer. There will be discussion of the manufacturing processes, ULE® selection, and
handling.
The next generation of large ground based astronomical telescopes will have their primary mirrors, and in
some cases secondary mirrors, built using a segmented approach. Corning has the capacity and capability to
produce mirror segment blanks from Corning ULEThe next generation of large ground based astronomical telescopes will have their primary mirrors, and in
some cases secondary mirrors, built using a segmented approach. Corning has the capacity and capability to
produce mirror segment blanks from Corning ULE® titania-silica glass in segment sizes ranging from 1.0-
meter to 1.8-meters flat to flat (1.2-meter to 2.1-meter point to point). Corning also has the capability of
producing large monolithic mirrors for use in secondary, tertiary and/or other mirror blanks up to 8.5-meters
in diameter. This paper will review and further discuss the material and processes employed by Corning to
produce several hundred to several thousand mirror segment blanks for extremely large telescopes, along
with large monolithic mirror blanks for downstream optics, titania-silica glass in segment sizes ranging from 1.0-
meter to 1.8-meters flat to flat (1.2-meter to 2.1-meter point to point). Corning also has the capability of
producing large monolithic mirrors for use in secondary, tertiary and/or other mirror blanks up to 8.5-meters
in diameter. This paper will review and further discuss the material and processes employed by Corning to
produce several hundred to several thousand mirror segment blanks for extremely large telescopes, along
with large monolithic mirror blanks for downstream optics.
KEYWORDS: Manufacturing, Mirrors, Glasses, Inspection, Large telescopes, Ultrasonics, Telescopes, Temperature metrology, Astronomical telescopes, Space telescopes
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.
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