Details:
◦ Gas-Fusion™ Process
◦ Stiffness of Gas-Fusion™ Mirrors
◦ Thermal Considerations
◦ Optical Capability
Gas-Fusion™ Process
No material (such as frit or fluxing agents) are used to manufacture the blank other than Schott Borofloat® (face sheets) and Duran® (tubing for the ribs). This allows the formation of true glass-to-glass seals both at the face plate/tube joints and at the tube/tube rib joints. Because of this exceptional fusion method, the bonds are 100% and cannot separate. In effect the individual glass components become a single monolith after our process.
This process forms a plano substrate. In order to achieve a needed radius of curvature, the blank is positioned on a concave/convex refractory mold (depending on need). The blank and mold are re-heated and the glass slumps near-net-shape around the mold. This eliminates the added step of an optical shop generating the curve.
Stiffness of Gas-Fusion™ Mirrors
Light weight mirror blanks are as stiff as a solid. At HEXTEK, we conducted an experiment using Gas-Fusion blanks of 40% and 16% density. The blanks were simply supported and a vacuum was pulled from the back side. We measured the front surface deflection using a 12″electronic spherometer. We found deflection under the mirror’s own weight was not measurable. A vacuum of 1.6 psi was applied which translated to a load of 37 times the weight of the 18″ mirror. The table below represents our data:
| 18″ Mirror – 16% Density, 3.5″ Thick, 0.25″ Thick Face Plate | ||||
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| Analytical Light Weight Value | Measured Value | Analytical Solid Value | ||
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| Weight | 12.5 lbs. | 11.4 lbs. | 70 lbs. | |
| Areal Density | .049 psi | .045 psi | .274 psi | |
| Self Weight Deflection | 4.3 µ-inch | 3.3 µ-inch | 4.0 µ-inch | |
| Max. Applied Load | 1.68 psi | 1.68 psi | 1.68 psi | |
| Deflection over 12″ | 55 µ-inch | |||
| Extrapolated Full Dia.Deflection | 124 µ-inch | |||
| Max. Calculated Bending Stress at 1.68 psi | 85 psi | |||
| Load Ratio of Areal Density | 34.3 | 37.3 | 6.13 | |
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| 15″Mirror 40% Density, 3.0″ Thick, 0.375″ Thick Face Plate | |||||
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| Analytical Light Weight Value | Measured Value | Analytical Solid Value | |||
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| Weight | 15.7 lbs. | 17.0 lbs. | 42.7 lbs. | ||
| Areal Density | .089 psi | .096 psi | .242 psi | ||
| Self Weight Deflection | 2.4 µ-inch | 1.9 µ-inch | 2.5 µ-inch | ||
| Max. Applied Load | 1.68 psi | 1.68 psi | 1.68 psi | ||
| Deflection over 12″ | 21 µ-inch | ||||
| Extrapolated Full Dia.Deflection | 33 µ-inch | ||||
| Max. Calculated Bending Stress at 1.68 psi | 85 psi | ||||
| Load Ratio of Areal Density | 18.9 | 17.5 | 6.94 | ||
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Thermal Considerations
The following are excerpts from a paper “Thermal Response of a Lightweight Gas-Fusion Mirror” published in SPIE Proceeding Vol.1994.
The results shown in this paper illustrate the rapid response of lightweight mirror blanks to small thermal perturbations.
A HEXTEK Gas-Fusion mirror (18″ in diameter, 3.5″ thick with a radius of curvature of 96″) was polished to a sphere. The mirror was supported on three points at about the 7/10 zone on the back plate. The mirror was positioned face up under a digital phase measuring interferometer located at the center of curvature. A 660 ohm resistor was attached to the center of the back plate using thermally conductive grease. At t = 0, the resistor was connected across a 110 volt AC source and the interferometer was set to capture an interferogram each minute. The thermal load was applied for 14 minutes and data was taken for another 24 minutes.
We have to bare in mind that this experiment addresses a worse case scenario with an athermal load applied to one portion of one face plate. The graph below indicates the figure change over time starting with no heat load, applied heat load and no heat load.
The three isometric views following show the surface figures at a) t = 0, 0.085 waves rms, 0.62 waves p-v, b) t = 14, 0.067 waves rms, 2.27 waves p-v, c) t =38, 0.085 waves rms, 0.62 waves p-v.
Results
Results indicate for this blank, with a face sheet of ~ 3/8″ thick, the thermal time constant is about 24 minutes. The thermal time constant is essentially independent of cte (coefficient of thermal expansion), only the magnitude of the deformation depends on the cte. In fact, for lightweight mirror structures, cross sectional thickness of the glass is the primary factor in determining thermal time constant.
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Optical Capability
Hextek Gas-Fusion™ mirrors are capable of being polished to high precision tolerances without print through issues.
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Routinely polished to diffraction limited imaging in the visible.
◦ Documented figure accuracies in excess of l/20 p-v, l/100 rms in the visible on plano mirrors.
◦ Fast f/0.5 one meter spheres polished to l/20 rms.
◦ Traditional polishing and finishing methods apply including Ion-figuring.
Print-through
Gas-Fusion™ mirrors have been precision polished without print-through issues for over 17 years. The surface deformations caused by print through are a function of the mirror structural design, degree of lightweighting, polishing force, and optical finish. Like all lightweights, some degree of print-through is measurable at some level, but if polished correctly, print-through is not meaningful.
Reliability
Hextek Gas-Fusion™ mirrors hold optical surface finishes well over time because they are 100% fused at high temperatures then run through a fine anneal cycle during the fusion process.
Coatings
Gas-Fusion™ mirrors readily accept all metallic coatings and high efficiency dielectric stacks.
