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.
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.
||Plate Thickness (mm)
||Rib Thickness (mm)
||Total Mass (kg)
||Areal Density kg/m2
Below are images of Hextek substrates that demonstrate readily achievable areal densities and they all have a diameter to thickness ratio of 6:1: