1. Département de Physique, Génie Physique et Optique, Centre d'Optique, Photonique et Lasers, Université Laval, Québec, G1K 7P4, Canada
   
2. Steward Observatory, The University of Arizona, 933 N. Cherry Avenue, Tucson, Arizona 8575, USA
   
3. Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, British Columbia V6T 1Z1, Canada
   
4. The QUILL Centre, The Queen's University of Belfast, Stranmillis Road, Belfast BT9 5AG, UK
   
5. Office of the Director, NASA Ames Research Center. Moffett Field, California 9403, USA

Correspondence to: Ermanno F. Borra¹ Correspondence and requests for materials should be addressed to E.F.B. (Email: borra@phy.ulaval.ca).

An optical/infrared telescope of 20–100 m aperture located on the Moon would be able to observe objects 100 to 1,000 times fainter than the proposed next generation of space telescopes¹. The infrared region of the spectrum is particularly important for observations of objects at redshifts z > 7. The apparent simplicity and low mass of a liquid mirror telescope, compared with a traditional pointable glass mirror, suggest that the concept should be considered further. A previously proposed liquid mirror telescope, based upon a spinning liquid metallic alloy², is not appropriate for infrared applications, which will require a liquid below 130 K. Here we report the successful coating of an ionic liquid with silver. The surface is smooth and the silver coating is stable on a timescale of months. The underlying ionic liquid does not evaporate in a vacuum and remains liquid down to a temperature of 175 K. Given that there are 10⁶ simple and 10¹⁸ ternary ionic liquids, it should be possible to synthesize liquids with even lower melting temperatures.