Properties of various mirror substrate materials |
| by Bryan Greer |
| Originally posted Oct. 1999. Last updated May 5, 2003 |
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Properties of various mirror substrate materials
Adventures in collimation
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refractive index, (near 550 nm)
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density, g/cm3
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Young's modulus, Gpa
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hardness, Knoop (kg/mm2)
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coefficient of thermal expansion, 1/ºC
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thermal conductivity,
W/(m ºC) |
specific heat, kJ/(kg ºC)
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Pyrex (7740)
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1.474
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2.23
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65.5
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418
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32.5 x 10-7
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1.13
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0.75
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Fused Silica (Quartz)
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1.459
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2.2
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73
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590
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5.5 x 10-7
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1.38
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0.74
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Plate glass
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1.520
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2.5
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73
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470
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86 x 10-7
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0.75
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0.73
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Astrositall
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opaque
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2.46
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90.2
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-
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±0.6 × 10-7
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1.18
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0.92
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Zerodur
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1.54
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2.52
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90.2
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550
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-0.2 x 10-7
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1.63
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0.85
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BK-7
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1.517
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2.51
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81.5
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510
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71 x 10-7
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1.13
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0.85
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Implications of these properties for a telescope's mirror
Refractive index: This is a measure of how much the material can slow down light (compared to a vacuum). It's a critical property for lens design, but is unimportant for reflective optics.
Density: This is how heavy the material is for a given volume. We usually want our optics as lightweight as possible, so a lower value is better.
Young's modulus: This is how stiff the material is. To minimize flexure in the optical cell, or to permit the thinnest possible optic for a given cell design, a higher value is better.
Hardness: Harder materials generally polish smoother at the microscopic scale, so a higher value is better.
Coefficient of thermal expansion: This describes how much the material will grow (volumetrically) with an increase in temperature. Since we want the shape of our mirror to remain the same while it's changing temperature, a low value is desired.
Thermal conductivity: This describes how quickly heat can be transported through the material. Since we want the mirror to cool quickly and track the falling ambient temperature closely, a higher value is better.
Specific heat: This is how much heat energy the material can store. A low value is desired, since this will help the mirror to adapt to a change in temperature more quickly.
Cost: This property isn't in the table above, but it certainly can be important!
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