English: An optical flat that has been coated with aluminum by vacuum deposition, to make a first-surface mirror. The coating is enhanced with dielectric coatings, both improving the reflectance and protecting the soft aluminum from damage. Both sides of the optical flat were certified to be polished to ${\displaystyle \lambda }$/20. That's 1/20 of the wavelength of the helium-neon laser used in the interferometer, which equates to a surface deviation of no more than 31.6 nanometers. The substrate is quartz, measuring 1 inch in diameter by 1/2 inch thick.