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Areal Density

Areal density is a term that relates the mass per optical surface area of a mirror; most often expressed in units of kg/m2. This parameter is of particular interest for optical payloads to be flown in space. For example, the 2.4 meter Hubble Space Telescope has an areal density of 180 kg/m2. Although the mirror was a glass lightweight egg-crate style design, the areal density is considered large by today's standards.

Density Chart

In contrast, the James Webb Space Telescope primary mirror, a composite of segments made from beryllium are at around 50 kg/m2. Today's customers want larger mirrors with low areal densities below 50 kg/m2. By nature, Hextek Gas-Fusion substrates have low areal densities. Below is a table for typical mirror sizes that shows the degree of mass reduction standard Hextek mirror blanks can achieve.

 Diameter Plate Thickness (mm) Rib Thickness (mm) Total Mass (kg) Areal Density kg/m2
250 mm 9 6 2.7 54
500 mm 9 6 12.8 72.4
1000 mm 11 6 66.4 84.5
1500 mm 13 6 177.4 100.4

Below are images of Hextek substrates that demonstrate readily achievable areal densities and they all have a diameter to thickness ratio of 6:1:


250mm Ultra Lightweight Blank

250mm Ultra Lightweight Blank

Front Face Plate Thickness: 3.0mm
Back Plate Thickness: 3.0mm
Rib Thickness: 1.0mm
Areal Density: 16 kg/m2

250mm Ultra Lightweight Mirror

250mm Ultra Lightweight Mirror

Front Face Plate Thickness: 1.5mm
Back Plate Thickness: 1.0mm
Rib Thickness: 1.0mm
Areal Density: 7.6 kg/m2

445mm Ultra Lightweight Blank

445mm Ultra Lightweight Blank

Front Face Plate Thickness: 4.0mm
Rib Thickness: 2.0mm
Mass: 3.8kg
Areal Density: 24.4 kg/m2

470mm Ultra Lightweight Blank

470mm Ultra Lightweight Blank

Front Face Plate Thickness: 4.0mm
Back Plate Thickness: 4.0mm
Rib Thickness: 1.5mm
Mass: 5.4kg
Areal Density: 31 kg/m2