Looking up at the inside of the Front-End Assembly lid, with the coldest of the five blocking filters in place. The filter is the circular bit in the middle, with some wrinkles in it. It's a very thin bit of polyimide plastic, coated with a similarly thin layer of aluminum, to let X-rays through but block visible, infrared, and ultraviolet light. The funny angled holes above and below the center are for inserting the calibration source. This is a small piece of radioactive material that emits X-rays of a particular energy, which are detected by a special pixel. Only the upper of the two openings is used for the calibration source. This lid is about 3 x 5 cm.
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