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1 WATER COOLED SPATIAL FILTER Field of the present development The present developments relates to a water cooled adjustable spatial filter. The filter can be used, for instance, for a laser system such as a CO2 laser. The laser system may be included in, or associated with, a lithographic apparatus or other equipment related to semiconductor processing. Related art Spatial filters may be included in laser systems. For instance, a spatial filter (SF) may be included in between respective beam tubes feeding an interferometer or other laser beam fed optical system. The spatial filter may be mounted onto a frame, fixing the filter to other equipment. The assembly may feature a mount to which the housing of the spatial filter is attached. The function of the spatial filter may be to ‘filter’ incident light. Herein, a small hole may be centred on the focal point of the incid...