Manufacturing Process of the Hubble Space Telescope s Primary Mirror

Transcription

1 Kirkwood 1 Manufacturing Process of the Hubble Space Telescope s Primary Mirror Chase Kirkwood EME 050 Winter /11/2017

2 Kirkwood 2 Abstract- The primary mirror of the Hubble Space Telescope was a 2.39-meter-wide paraboloid made from ultra-low expansion glass formed into a 0.33-meter-thick, 770-kilogram honeycomb composite coated in aluminum and magnesium fluoride. Due to the expected working conditions of the mirror, its size, and the narrow tolerances on its curvature and surface finish, both traditional and specialized manufacturing methods were used to ensure adequate surface finish and physical properties. Each of the traditional methods performed within their expected dimensional tolerances. The specialized methods however, were used when tighter tolerances were needed, and during the production of special materials. Through the combination of these processes, the Hubble Telescope s primary mirror achieved a final tolerance of ±32 nanometers from its desired curvature, while also possessing a coefficient of thermal expansion less than 1/100 th that of ordinary glass. Introduction- The primary mirror of the Hubble Space Telescope was made from a circular sheet of ultra-low expansion glass composite with a diameter of 2.39 meters. It had a shallow concave face coated with aluminum and magnesium fluoride, enabling it to reflect a large spectrum of light. The manufacturing process began with the casting of two ultra-low thermal expansion (ULE) glass plates which were infused with titanium oxide (TiO 2) through a process called flame hydrolysis. Following this, the two plates were fused with a ULE glass honeycomb core, forming a 0.33-meter-thick glass composite. The front face of this composite was then shaped into a shallow concave paraboloid using diamond-abrasive face grinders. After this came a large amount of conventional abrasive polishing, followed by CO 2 laser polishing to achieve the mirror s final surface finish. The last step in the manufacturing process was the application of the aluminum and magnesium fluoride coatings using vacuum deposition. Process Analysis- The production of the two glass plates mentioned previously was performed by Corning Incorporated, who are best known for their shatter resistant glassware that is a variant of the glass used in the Hubble Telescope s mirror. This ultra-low expansion glass, as trademarked by Corning, is similar to normal glass, in that it is largely composed of silica (SiO 2). However, what distinguishes ULE glass is the presence of TiO 2 throughout its structure, which grants it extraordinary resistance to thermal expansion and stress, when present in concentrations less than 10%. When compared, ULE glass has a coefficient of thermal expansion that is roughly a

3 Kirkwood 3 hundredth that of pure silica glass. [1] ULE glass is produced by melting normal silica pellets, however they are heated using flames that are injected with chemicals which are reduced into TiO 2 in a chemical process known as flame hydrolysis, and this titanium dioxide is then infused into the silica melt, as shown in Figure 1. Due to its high melting temperature and similarly high viscosity, ULE glass generally cannot be poured like normal glass. This fact requires that ULE glass be smelted in its mold. In the case of the Hubble Telescope s glass plates, the silica pellets were placed into a flat, circular mold, which were then melted by the infused flames. Once the chemical composition of the glass reached its desired levels, it was cooled relatively quickly to ensure that it developed the amorphous structure characteristic of glasses. [2] Figure 1: ULE Glass Casting [2] The next major step of the mirror s construction involved the assembly of the ULE glass composite. However, prior to being assembled, the honeycomb ULE glass core was manufactured and cut to size. The hexagonal cells of the core were assembled one by one by fusing thin ULE glass elements together to form each cell. [3] This entire production process had to be conducted at an elevated temperature to aid the fusion of the elements, and to prevent any thermal contraction. After the core was complete, the two glass plates were introduced to the high-temperature environment, after which they were fused to each side of the core in a manner similar to that shown in Figure 2, thereby forming the final composite structure. Even though the assembly had been completed, the ULE glass composite still endured a threemonth cooling process, bringing its temperature from 1200 degrees Celsius down to room temperature at roughly 20 degrees Celsius. The purpose of the extended cooling process was to ensure that no cracks or imperfections developed in the composite s structure. The difficultly of producing the composite payed off however, resulting in an 80% reduction in weight over that of a solid glass disk of the same dimensions. [4] [3]

4 Kirkwood 4 Figure 2: ULE Glass Honeycomb Composite [3] The unmodified glass composite disk was then subjected to a number of face grinding operations using a series of diamond abrasive tools with gradually decreasing grain size. While diamond is a common abrasive for precision grinding applications, it was specifically selected for the Hubble Telescope mirror because of its unique behavior when grinding glass. In short, diamond abrasive grains generate far less surface heat while grinding glass than other common abrasives such as silicon carbide, which is the most commonly used abrasive for glass. [5] When a diamond grain cuts into a glass surface, there is a short period of viscous deformation before brittle fracture occurs in the glass. [6] The fracture occurs when the shear stress in the glass exceeds its tolerance, after which the diamond grain proceeds forward with no more obstructions. Since there is minimal friction and little work done by the grain, the heat imparted to the surface is quite small. Diamond behaves this way since the grains tend to have very sharp edges when compared to those of silicon carbide. These sharp edges cause the increased stress concentrations. The downside to using diamond grains however, is that they create a comparatively rough surface due to the brittle fractures that occur on the surface, as shown in Figure 3. However, since the grinding process for the Hubble Telescope s mirror was followed by several polishing stages, the rough surface finish was of little consequence. After the grinding process was completed, the mirror was subjected to a long series of basic abrasive polishing operations which smoothed the surface of the mirror and removed approximately 0.75mm from the whole surface of the mirror. [4] The polishing was done using manually-controlled tools and by hand. Each round of polishing gently compressed and smeared any rough surfaces along the mirror s surface, slowly moving it closer to its completed state. [6]

5 Kirkwood 5 Figure 3: Surface Finish of Diamond and Silicon Carbide Abrasives [6] Following the physical polishing came laser polishing to make final adjustments to the mirrors shape, and to ensure an extremely smooth finish. A high-powered CO 2 laser with a wavelength greater than 5μm was moved across the mirrors entire surface several times. At the point where the laser contacted the surface, the thin, top layer of glass softened and melted, allowing it to freely settle into a more level state. The laser only interacted with the top layer of glass, because glass becomes extremely absorbent of light with wavelengths greater than 5μm. [7] Therefore, almost all of the laser s energy was absorbed by the top layer of the mirror, smoothing the surface to a final tolerance of ±32nm of its desired curvature. [4] The final step in the production of the Hubble Telescope s mirror was the application of its reflective coatings. Both coatings were applied using vacuum deposition, which involved placing the blank Hubble mirror into a vacuum chamber that was equipped with evaporators responsible for vaporizing the coating material. Once the air was evacuated from the vacuum chamber, the evaporators ejected the gaseous coating into the chamber, after which began to condense on the blank mirror as shown in Figure 4, as well as on any other surfaces which were cool enough to attract the vapor. Hubble s mirror received two different coatings. The first was a 75-nanometer thick layer of aluminum, which was the primary reflection material. The second coating was a 25-nanometer thick layer of magnesium fluoride, which served as both a protective coating for the aluminum, and as an enhancer for the reflection of ultraviolet light. [8]

6 Kirkwood 6 Figure 4: Vacuum Deposition [9] Conclusion- At the time of its launch, the Hubble Space Telescope was, and to this day remains, a technical marvel. Every step of its construction required extremely high precision, as evidenced by the twenty-year design and assembly process. Numerous parts had to work in tandem so that Hubble could complete its mission. Of all these parts, the most important was the primary mirror, which was responsible for collecting and concentrating any incoming light. Production of this mirror alone took nearly a decade, with extreme care having been taken during each step of construction.

7 Kirkwood 7 Works Cited [1] Corning Incorporated, "ULE Ultra Low Expansion Glass," 13 October [Online]. Available: [2] K. Hrdina, "Production and properties of ULE glass with regards to EUV masks," in Proceeding at the international workshop on extreme ultra-violet lithography, New York, [3] J. Spangenberg-Jolley and T. Hobbs, "Mirror substrate fabrication techniques of low expansion glasses," in International Congress on Optical Science and Engineering, New York, [4] "Chapter 5: Hubble," [Online]. Available: [5] S. Kalpakjian and S. R. Schmid, Manufacturing Engineering and Technology, 6 ed., New York: Prentice Hall, [6] S. Malkin, "Grinding of glass: the mechanics of the process," Journal of Engineering for Industry, pp , [7] F. Vega and F. Laguarta, "Laser application for optical glass polishing," Optical Engineering, vol. 37, no. 1, pp , [8] "Nuts and Bolts: Optics," [Online]. Available: [9] U. Hiroaki, "Physical Vapor Deposition," Tokyo University of Agriculture and Technology, Tokyo, 1997.