A new ultralow-expansion, modified fused-silica glass.

A new modified fused-silica glass whose coefficient of thermal expansion at 20 degrees C is less than 1/25 that of fused silica is described. It should be particularly useful for large, first-surface mirrors and in other applications where extremely low expansion is desirable. Both solid and lightweight telescope mirror blanks can be constructed using fusion techniques. A discussion of a number of physical properties is included.