The most common type of beam expander is derived from the Galilean telescope which usually has one negative input lens and one positive output lens. The input lens presents a virtual beam focus at the output. For low expansion ratios (1.3-20 times) the Galilean telescope is most often employed due to its simplicity, small package size and low cost. * Material: BK7 fine annealed * Diameter: -0.1mm * Thickness: ¿¬+0.1mm * Flatness: l/4 * Surface Quality: 60-40 scratch-dig * Coating: HR dielectric coatings R > 99%.