Specifications Material: BK7 Grade A optical glass Dimension Tolerance: +/- 0.25mm 90° Deviation Tolerance: Standard series: < 30 arc seconds Precision series: up to 10 arc seconds Flatness: Standard series: l/2 at 632.8 nm Precision series: up to l/4 at 632.8 nm Reflectivity: R > 95% per face from 630 to 680 nm Surface Quality: 60-40 scratch and dig