An optical arrangement 1, wherein four directional mirrors are provided, each of said four mirrors being mounted for rotation on an edge thereof to form a quadrangle around the optical axis of the concave mirror, each of said mirrors creating {HEAD ~ an optical effective curtain coacting} to surround {TAIL ~ the space} being mounted 
An optical arrangement 1 , wherein four directional mirrors are provided , each of said four mirrors being mounted for rotation on an edge thereof to form a quadrangle around the optical axis of the concave mirror , each of said mirrors creating { HEAD ~ an optical effective curtain coacting } to surround { TAIL ~ the space } being mounted
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