The Earthshine Telescope Project

Abstract:
The Earthshine telescope project is a collaborative effort between Lund Observatory (LO) in Sweden and the Danish Meteorological Institute (DMI) with the purpose of constructing one or more robotic telescopes to record the albedo of the Earth over a long time. The objective is to measure long-term development of the global cloud cover and reflectivity for climate modeling. A 1% change in the Earth's albedo results in an average temperature change of 0.5 K which calls for high precision in the albedo measurements. This poses strict demands on the telescope design, in particular with respect to suppression of straylight. The paper describes our proposed optical and mechanical design of the Earthshine telescope, and presents a preliminary straylight analysis of the design as well as first steps towards an 'error budget' for the system. Polarisation due to light-path folding could be an issue for the design being studied.