Low Expansion Overview

Low expansion optical materials, as their name implies, are known for an impressively low coefficient of thermal expansion and are very resistant to high-temperature thermal shock. Typically, low expansion materials have a near zero coefficient of thermal expansion. Ceramic glass substrates are generally engineered to retain their mechanical and optical properties – especially reflected wavefront – even during extreme temperature changes.

The material has very high mechanical strength, high chemical durability/resistance, and superior machinability characteristics. These materials can be available in sizes up to 8 meters in diameter with the bulk of the mirror substrates ranging in the in the 1 – 2 meter range.

Characteristics of low expansion materials include:

  • Extremely low coefficient of thermal expansion (CTE)
  • Large operating temperature range and shock resistivity
  • Extreme hardness, fracture toughness
  • Permeability
  • Excellent chemical stability
  • Non-porous material
  • High affinity for coatings
  • Few inclusions and bubbles
  • Large size capabilities

Low expansion materials are ideal for astronomical telescope mirror substrates, lightweight mirrors & mounts, laser cavities, and precision metrology fixtures. In fact, these materials are the material of choice for all of the world´s segmented telescopes currently in operation.

Sydor can manufacture wafersmirrorswedge prismswindows, and flats using low expansion materials.


Based upon customer demand, Sydor Optics offers Low Expansion made from the following substrate materials. Of course, if you don’t see a material you need, please contact a Sales Engineer, who will gladly discuss additional options that may be available.