Why does New Focus use Pyrex as the substrate?
We chose Pyrex for our substrates because it offers a lower thermal expansion coefficient than BK7 optical glass and polishes more easily than fused quartz. Because Pyrex has inhomogeneities in its refractive index, these mirrors are not suited for transmissive applications. We fine-grind the backside substrates to prevent inadvertent transmissions. All the edges are chamfered to avoid chipping during use.
