A Dual-Beam Design Concept for the LOUPE Spectropolarimeter

In this thesis, a dual-beam design concept for the LOUPE (Lunar Observatory for Unresolved Polarimetry of Earth) spectropolarimeter is presented based on the work by Karalidi [2012] and Hoeijmakers [2013]. The mission statement of LOUPE is: LOUPE shall do spectropolarimetry of the Earth from the Moon in order to obtain reference data for exoplanet research. The requirements made by Karalidi [2012] and Hoeijmakers [2013] have been extended to a complete quantifyable requirements list from which the actual instrument can be designed. Trade-offs have been made for the choice of the technique to do spectropolarimetry and the polarizing beamsplitter that need to be implemented in LOUPE. The trade-off for the spectropolarimetric technique resulted in the choice of the spectral modulation technique in the form of the SPEX technique [Snik, 2009], because it allows for a compact design without moving components and is capable of doing instant polarization measurements. However, the SPEX technique does not allow for measuring circular polarization, meaning that LOUPE will only be capable of doing full linear spectropolarimetry. The trade-off for the polarizing beamsplitter resulted in the choice for the Savart plate as the beamsplitter, since a Savart plate allows for uniform splitting over the entire field of view. Using a ZEMAX model of LOUPE, it was shown that the aberrations caused by the Savart plate can be solved using a cylindrical lens. The dual-beam concept for LOUPE was determined in this thesis to be an f/4 system that has a field of view of 20 deg. The quarter-wave plate and multi-order-wave plate required for SPEX are located directly after the objective lens. The Savart plate is located in the image plane of a hexagonal microlens array (MLA). The MLA splits up the field of view in 44x35 pieces. Using clever stacking, the two modulated spectra for each MLA element are imaged on a 1024x1024 pixels detector. This results in 3x3 samples over the Earth, which allows for spatially resolving continents and oceans. A Matlab model for calculating the reflected solar flux levels on the Moon was made to analyse the obtainable polarization sensitivity per sample. This showed that with this concept a polarimetric sensitivity of 10^-4 can be achieved for terrestrial phase angles around 0 deg. A polarimetric sensitivity of 10^-3 can be achieved for terrestrial phase angles around 170 deg.Space Systems EngineeringAerospace Engineerin