Custom-engineered sapphire windows that can operate from full vacuum to 10,000 psi, depending upon the configuration, and from cryogenic to 1000 °C while resisting chemicals and particulates, are available from Meller Optics Inc. The sapphire viewport windows are manufactured to specification in sizes from 1/4 to 10 in. in diameter and from 1/2 mm to 1 in. thick, with stepped edges and elliptical edge-shaping, holes, slots and wedges for mounting, and with antireflection coatings. Featuring Moh 9 hardness, they are resistant to chemicals (fluorine to 300 °C) and most acids, and are impervious to water and to blowing dirt and sand. They are second only to diamond in terms of hardness. Precision-fabricated, they exhibit flatness to 0.5 fringes HeNe and parallelism from 20 to 2 arcsec.