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Borosilicate Optics

At Hextek Corporation of Tucson, Arizona, we use Schott Borofloat 33® and Duran® borosilicate glass for the production of lightweight mirror blanks. Though we can cast substrates, the low working temperatures of this glass allows the use of our Gas-Fusion process to fabricate a sandwich-structured mirror blank. Our substrates have been finished into mirrors and are currently being used as optics in world class telescope systems including:

» The Large Binocular Telescope » Sloan Digital Sky Survey » MMT

» ARC 3.5 Meter Telescope

>>LCOGT One Meter Telescopes

» Subaru » Lowell Observatory » Keck I

Borosilicate Material

The original idea behind developing borosilicate glasses about 100 years ago was to have a durable material with a low enough coefficient of thermal expansion to survive rapid changes in temperature without cracking or breaking. Borosilicate glass consists of 70% pure silica, 10% boron oxide, 8% sodium oxide, 8% potassium oxide and 1% calcium oxide. High silica content translates to a lower thermal expansion coefficient of the glass. The other constituents that have been added to the silica help reduce the melting temperature of the glass. Pure fused silica SiO2 has a softening point of 1600°C while typical borosilicate glasses are closer to 800°C. Because borosilicate glasses have such low working temperatures, this allows for an easier opportunity to melt and fuse the glass. This ability to cast the glass at a reasonable temperature range coupled with a low thermal expansion is the reason why the material was chosen as the primary mirror substrate in the Mount Palomar 200-Inch and Lick Observatory 120-Inch Shane telescopes.

Today's Uses

Although borosilicate glass does not receive the respect it deserves as an optical material because it is not a “zero coefficient of expansion” material, its inexpensive raw material cost and low working temperatures make it ideal for large optical systems with thermal management.  In recent years, there has been a revival of borosilicate glass as a mirror substrate after the development of lightweight spin-cast technology by the University of Arizona's Steward Observatory Mirror Lab. Use of this material continues as a primary mirror substrate with the production of seven 8.4 meter borosilicate mirrors for the Giant Magellan Telescope by the Mirror Lab.  Other telescope systems include:

» The Large Binocular Telescope (2 x 8.4 Meters) » Magellan I and II (6.5 Meters)
» Large Synoptic Survey Telescope (8.4 Meters) » MMT (6.5 Meters)

Borosilicate Glass Specifications

Mechanical Properties

» Density (25°C) ρ 2.2g/cm3
» Poisson's Ratio µ 0.2
» Bending Strength σ 25 MPa

Thermal Properties

» Coeff. of Linear Thermal Expansion a (20-300°C) 3.25 x 10-6/°K
» Specific Heat Capacity cp (20-100°C) 0.83 KJ/kg°K
» Thermal Conductivity λ (90°C) 1.2 W/m°K

Optical Properties at λ = 589.3 nm

» Index of Refraction n 1.47133 » Refractive Index nd (λ=587.6nm) 1.47140 » Stress Optical Coefficient K
4.0 x 10-6 mm2/N
» Abbe Constant ve=(ne-1)/(nf′-nc') 65.41 » Dispersion nF-nC 71.4 x 10-4

Available Standard Sizes

Borofloat 33® Sheet Glass

Sheet Glass

» Standard Size: 2300mm x 1700mm
» Special Order Size: 2300mm x 2300mm
» Plate Thickness Range: 0.7 - 25.4mm

Duran Tube Sizes

Duran Tubes

» Standard Diameter Range: 3mm - 325mm
» Standard Wall Thickness: 0.7 - 10mm
» Maximum Tube Length: 1500mm