Properties of various mirror substrate materials

by Bryan Greer
Originally posted Oct. 1999. Last updated May 5, 2003

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Telescope Thermodynamics

Sept. 2000 Sky & Telescope magazine companion web site
May & June 2004 Sky & Telescope magazine companion web site
Using fans with a Newtonian telescope
Tips on attaching a temperature probe to your telescope

Optical Miscellany

Try this at home!
How atmospheric seeing affects telescopes with different focal ratios
Animated focal plane illumination map
Properties of various mirror substrate materials
Adventures in collimation
refractive index, (near 550 nm)
density, g/cm3
Young's modulus, Gpa
hardness, Knoop (kg/mm2)
coefficient of thermal expansion, 1/ºC
thermal conductivity,
W/(m ºC)
specific heat, kJ/(kg ºC)
Pyrex (7740)
1.474
2.23
65.5
418
32.5 x 10-7
1.13
0.75
Fused Silica (Quartz)
1.459
2.2
73
590
5.5 x 10-7
1.38
0.74
Plate glass
1.520
2.5
73
470
86 x 10-7
0.75
0.73
Astrositall
opaque
2.46
90.2
-
±0.6 × 10-7
1.18
0.92
Zerodur
1.54
2.52
90.2
550
-0.2 x 10-7
1.63
0.85
BK-7
1.517
2.51
81.5
510
71 x 10-7
1.13
0.85

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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