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.
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.
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.
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.
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.
The Airborne Laser (ABL) program requires a large aperture, highly transparent window to allow the high energy laser beam to be focused on targets. This window presents many challenges as it is thin, large in diameter and very highly curved. Additionally, the window must be made from a material highly transparent at 1.315 micrometers, the chemical oxygen-iodine laser wavelength, have good transmission from the visible through 3 micrometers and be able to withstand the rigors of operations on a tactical aircraft. To manufacture this window, a unique partnership between two companies, Heraeus and Corning, was forged to demonstrate the process and manufacture the window blanks. Infrasil 302, a Heraeus product, is the only material with low absorption at 1.31 micrometers that can be produced in large enough quantities to make a window blank of the required size. Corning has developed the technology to flow- out and sag glass products to make highly curved optics without the need to machine them out of a cylindrical block. Using their experience and a common desire to support the ABL program, the two companies worked together to develop the processes that produce the window blanks. Contraves Brashear Systems of Pittsburgh will polish the blank in to its final form, with coatings applied by Optical Coating Laboratories, Inc. of Santa Rosa to maximize transmission.
Fabrication of lightweight mirrors from low expansion glasses can be achieved using several techniques. Simple machining can reduce the weight of a typical solid mirror blank by up to 50 percent. Even more weight reduction can be realized through a sandwich type assembly of a lightweighted central core and two faceplates. The central core can be fabricated by cutting out cells form a solid blank using traditional fixed abrasive diamond grinding or abrasive waterjetting technology, or by fusion bonding thin struts into a honeycomb structure. The resultant lightweight core can then be bonded to mirror faceplates using either a fusion or frit bonding process. Each mirror fabrication approach offers its own advantages and disadvantages in term so weigh reduction, design flexibility, manufacturing time, and cost. These factor will be discussed, along with Corning's current size capability for each techniques.
ULETM titania-silica binary glass is being used for the primary, secondary, and tertiary mirrors for the Subaru 8- meter Japanese National Large Telescope. The primary 8.3- meter mirror blank was made by Corning Incorporated in 1994, and is in the final stages of optical polishing at Contraves Brashear Systems. Corning has also manufactured two of three lightweight secondary mirror blanks to date, and has delivered two lightweight tertiary blanks. These lightweight mirrors were designed and analyzed by Mitsubishi Electric Corporation and Contraves Brashear Systems to meet performance requirements. This paper describes key design criteria for the chopping secondary mirrors, including finite element analysis results. The manufacturing process used by Corning is also described.
ULETM, the titania-silica binary glass with zero expansion coefficient, is an ideal material for large telescope mirror blanks due to the unique combination of its optical, thermal and mechanical properties--together with the ease of fabrication--which help meet performance and durability requirements in a cost-effective manner. Indeed, the 8m class Subaru and Gemini telescope mirror blanks have been fabricated successfully from ULE glass and will be in full operation in the not too distant future. This paper will focus on the stringent reliability requirements which the mirror blank must meet during fabrication, transportation, installation and operation atop high mountains with extreme environmental fluctuations. In particular, the paper will present strength and fatigue data for ULE glass as a function of surface finish. Such data are critical for selecting the appropriate surface finish to ensure mechanical reliability of the mirror blank at various stages of fabrication and during transportation. The use of Weibull statistical distribution for surface flaws combined with Power Law fatigue model helps arrive at a safe stress level which should not be exceeded to ensure the mechanical reliability of the mirror blanks. The safe stress level is verified through independent static fatigue tests on ULE discs with surface finish identical to that of the mirror blank. In this manner the mechanical reliability of large ULE mirror blanks can be ascertained at extremely low failure probabilities. The successful application of reliability model to both Subaru and Gemini mirror blanks will be illustrated.
Corning Incorporated has successfully completed the manufacture of three 8-meter class mirror blanks, the Subaru 8.3-meter and the twin Gemini 8.1-meter blanks. These mirrors, constructed of ULETM zero expansion glass by the hex seal process, are nearing completion at the optical polishers and will soon be installed in their respective telescopes. This paper describes the manufacturing process for the mirror blanks, summarizes key engineering studies performed during these projects, and gives quality results for the three blanks. In all cases, the 8-meter mirror blanks met both the customer's quality requirements as well as schedule timelines. Other applications of this process are also given.
The manufacturing of 8-meter mirror blanks by Corning's hex seal process requires several handling steps, including lifting and turnover of both plano and contoured configurations. Special equipment was designed and built to lift and turn these 35-ton, 27-foot diameter pieces of glass. Finite Element Analysis was used to evaluate the lifting and turnover equipment designs and the handling processes. The analyses predicted the stress and distortion in the glass. The results showed that the equipment designs and processes would be safe; that is they would not stress the glass beyond the safe design stress limits.
The zero expansion titania-silica binary glass ULE™ offers an optimum combination of thermal, mechanical, and optical properties which make it an ideal material for precision optical structures and lightweight telescope mirrors. Its near-zero thermal expansion over the ambient operating temperature range helps preserve optical figure; the absence of hysteresis in its thermal expansion curve ensures dimensional stability of the mirror in extreme environments; its low density affords high specific stiffness thereby reducing elastic deformation of lightweight structures; its high fatigue resistance permits higher long-term stress without compromising mechanical reliability; and its excellent optical and birefringence properties facilitate inspection and quality assurance in terms of thermal expansion homogeneity, defect level, and residual strain.
This paper reviews the key physical properties of ULE glass and the various fabrication techniques available for making lightweight mirrors. Special emphasis is given to ULE's mechanical behavior which controls the long-term reliability of mirror blanks during fabrication, shipping, installation, and operation. It describes the 8-meter class mirror blank manufacturing process and the stress/time histories to which such blanks are exposed. These parameters, together with strength and fatigue data, were used to optimize surface finish for adequate strength and to evaluate an 8-meter blank support system to ensure long-term reliability of the blank during transportation. The paper concludes with recent advances and development programs in support of the manufacture of precision optical mirror blanks.
KEYWORDS: Glasses, Mirrors, Ultrasonics, Time metrology, Manufacturing, Temperature metrology, Telescopes, Data modeling, Surface finishing, Transducers
Improvements made in the ultrasonic thermal expansion measurement methods used at Corning Incorporated have significantly increased the precision of absolute CTE measurements on ULETM glass. Precise CTE data were required for modeling the performance of 8-meter mirror blanks. Several repeatability and reproducibility studies for many of the measurements performed throughout the 8- meter process were performed. Measurement equipment and procedures were then improved to obtain the required precision. A volume of absolute CTE data collected during the manufacture of three 8-meter mirror blanks is summarized, showing excellent results. Comparison with previously demonstrated precision is also given.
The Japan National Large Telescope (SUBARU) under construction adopted Corning ULE glass for its primary mirror, which is made from 44 hexagonal blank segments (hexes) sealed together. The optimized hexes placement pattern was determined using the simulated annealing method so as to minimize the thermal deformation due to the inhomogeneity of the coefficient of thermal expansion of the hexes. The sealing process has been successfully completed and now the flat-to-flat blank is under the grinding phase to be followed by the sag-down phase for meniscus forming. The present paper summarizes the hexes placement pattern optimization method and various analysis behind its actual application, including current status of the fabrication.
Coming Incorporated was awarded a contract in August 1991 by Mitsubishi Electric Corporation (Melco), Tokyo, Japan to fabricate an 8.31 meter diameter ULETMpnmary mirror blank for the Subaru (Japan National Large Telescope) Project. The project is sponsored by the National Astronomical Observatory of Japan. This paper reviews the progress and status of the project to date. Coming's history in producing mirror blanks for astronomy spans nearly 60 years beginning in 1936 with the manufacture of the 200 inch Hale Telescope blank made of Coming's borosilicate glass. Since that time, among the many mirror blanks made of Coming borosilicate, fused silica, and ULE glasses are such notable mirrors as the 3.6 meter ESO, 4.0 meter Canadian, 2.7 meter McDonald Observatory, 2.4 meter Hubble Space Telescope, and several 4.0 meter thin meniscus blanks. Coming ULE Ultra Low Expansion glass was the preferred material of choice for the Subaru primary mirror because of its highly homogeneous, zero thermal expansion coefficient.
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