For use in military navigation and targeting systems, Meller Optics Inc. has announced custom-fabricated large sapphire windows up to 18 in. in diameter by 1/4 in. thick. They offer Moh 9 surface hardness, transmitted wavefront to λ/4, parallelism to 2 arc sec and a 70:1 aspect ratio. Resistant to water, most acids, alkalis and harsh chemicals to ~350 °C, they have surface finishes to 60-40 scratch-dig. They are available in any quantity, from prototype through production.