In order to study the oldest and most distant objects in the universe, a 100 meter diameter Liquid Mirror Telescope has been proposed for deployment on the Moon. High redshift places the spectral region of interest of these objects in the 1 to 10 µm wavelength region. This requires that the optics be kept very cold, typically at or below 130 K, which precludes the use of the traditional liquid Mercury mirror. The ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate has been coated with Silver by Physical Vapor Deposition (PVD) under high vacuum, and shows good reflectivity properties while remaining liquid down to 175 K. An approximately 10 nm thick Chromium diffusion-stop layer previously deposited before the Silver further improves reflectivity. Both the Silver and the Chromium exist as a colloidal dispersion in the ionic liquid, rather than as a continuous solid layer.

Details of the process of depositing reflective layers on an ionic liquid by PVD are discussed, and consideration is given to the potential scale-up process for in-situ coating of a 100 meter ionic liquid mirror telescope on the Moon.