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The efficiency of the cameras can be split into two components, one due to the geometry of the cameras (e.g. size of the central obstruction) and one due to losses at each coated surface. For small grating angles, the geometric efficiencies of the 235 mm and 500 mm cameras are about 67 and 73 respectively. At large grating angles (greater than about 44 and 60 for the 235 and 500 mm cameras respectively), the geometric efficiency drops off rapidly due to overfilling of the grating. This will for example occur with the 2400B grating when used in the red.
Each camera contains two aluminium mirrors, with a reflectivity of about 85 , and four air/glass surfaces with single layer anti-reflection magnesium fluoride coating, each of which has a reflectivity of about 2 at 5500 Å. We therefore expect the 235 mm and 500 mm cameras to have total effective efficiencies of 45 and 49 respectively.