Photometric Evaluation and Calibration of the Earthshine Telescope

Abstract: An earthshine telescope has been designed and is being fabricated at Lund Observatory in collaboration with the Danish Meteorological Institute. The instrument will measure the albedo of the Earth by observing the Moon in white light and in certain spectral bands. The measurement is based on relative photometry of the dark and bright sides of the Moon, so the optical design of the telescope has been carried out with special attention to photometric quality and stray light performance. In addition, advanced operating modes have been developed. Hence, the instrument is expected to be superior to existing earthshine telescopes. The LO/DMI Earthshine Telescope was envisaged to provide produce albedo measurements with an accuracy on the order of 0.1%. The objective of this thesis was to evaluate the accuracy of some of the main elements that contributes to the final photometric performance of the Earthshine Telescope. In particular the performance of the shutter was analyzed, since it is very crucial to obtain accurate photometry of the observation, especially due to the high range of the exposures from 10ms to few minutes which is needed for the albedo calculation. Also the proper characterization of the spectral filters transmission is very important for the precision of the spectro-photometric analysis of the Earth’s albedo, and for this reason spectral measurements were done for all the filters of the Earthshine Telescope. It was found that for 10ms the accuracy of the shutter response is 0.4%, and this will affect the accuracy of some of the operating modes of the Earthshine Telescope that require short exposures times. Nevertheless it is possible to decrease the error in shutter response for these operating modes by reducing the amount of light falling into the detector using a neutral density filter or reducing the aperture. Also spectral characterization of all the color filters and Knife Edge Optical Density filters of the Earthshine Telescope was done, obtaining spectral curves that matched very well with the specified by the manufacturers.