Areal Density

 

Areal density is an often used 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 light weight egg-crate style design, the areal density is considered very large by today’s standards.  In contrast, the James Webb Space Telescope primary mirror, a composite of segments made from beryllium, are at about 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.  Note that these substrates have a diameter to thickness ratio of 6:1 and there are no central holes.

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

The above chart demonstrates Gas-Fusion substrates of “standard” design with no attempt to significantly reduce areal density.  The following are some examples of blanks made that demonstrate our ability to significantly reduce mass.

 

250-mm-ultralight-demonstrator-1

250mm Ultra Light Weight Blank

Front Face Plate Thickness: 3.0mm

Back Plate Plate Thickness: 3.0mm

Rib Thickness: 1.0mm

Areal Density: 16 kg/m2

 

 

Gas Fusion 2010 Back

250mm Ultra Light Weight Mirror

Front Face Plate Thickness: 1.5mm

Back Plate Plate Thickness: 1.0mm

Rib Thickness: 1.0mm

Areal Density: 7.6 kg/m2

 

 

445 mm ultra light

445mm Ultra Light Weight Blank

Front Face Plate Thickness: 4.0mm

Back Plate Plate Thickness: 4.0mm

Rib Thickness: 2.0mm

Mass: 3.8kg

Areal Density: 24.4 kg/m2

 

 

 

18 ultra frnt a

470mm Ultra Light Weight Blank

Front Face Plate Thickness: 4.0mm

Back Plate Plate Thickness: 4.0mm

Rib Thickness: 1.5mm

Mass: 5.4kg

Areal Density: 31 kg/m2