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Greater Collecting Area
The X-ray Multiple-Mirror Mission (XMM for short) required many mirrors, some with parabolic shapes and others with hyperbolic shapes. These mirrors were layered and placed on top of each other to help focus as many x-rays as possible. The large numbers of mirror and their size gave XMM the largest collecting area of any X-ray observatory ever built at the time of its launch in 1999.
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