Photometric Characterization of the Earthshine Telescope

Uncertainties in determination of the Earth’s albedo currently limit our understanding of climate and climate change. A new telescope has been designed and built in collaboration between Danish Meteorological Institute and Lund Observatory to study the ratio between the intensities of earthshine to moonlight. This ratio is proportional to the terrestrial albedo. The aim of the Earthshine Telescope is to acquire long-term albedo data with a precision of about 0.1%. In order to achieve this goal the precision of the intensity ratio must be of the same order of magnitude or less. Preferably the accuracy should be reduced to similar values. The aim of this master thesis has been to photometrically characterize the Earthshine Telescope. I have set up an error budget for both the earthshine and moonlight intensities that is relevant for the operational mode of the telescope which is currently in use, the Co-add mode. In Co-add mode both the dark and the bright parts of the Moon are observed simultaneously in a long series of short exposures that are subsequently aligned and co-added. The stacking of many frames allows the signal-to-noise of the earthshine to build up without over-exposure of the bright side of the Moon. The image reduction process has three different steps: Bias subtraction, flatfielding and removal of scattered light on the dark side of the Moon. The three steps have been analyzed separately in order to determine their individual contributions to the error on the intensities. It is possible to reach the required level of precision in the intensity ratio of 0.1% at least for some lunar phases around 140, close to New Moon. The main contributor to the error on the earthshine intensity is photon noise and the main contributor to the error on the moonlight intensity is the flatfield. The accuracy in the intensity ratio is typically a few percent, although it should be noted that this is an upper limit. The dominant source of systematic uncertainty is the imperfect removal of scattered light