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ACIS uses polyimide/aluminum meshless films placed above the two CCD arrays to filter optical and ultraviolet light, so that the CCDs see only X-radiation. It is necessary to calibrate these filters by mapping their soft X-ray transmission on fine spatial scales, so that the filter response can be removed from the CCD data and a more accurate estimate of the true sky recovered. Different OBFs are used for each array, with the ACIS-S OBF having a thinner aluminum coating, since light falling on the spectroscopy array will often be dispersed by transmission gratings which can be placed in front of the ACIS-S array.
We measured engineering and flight versions of these filters and witness samples of the filter material at the SRC between June 1995 and January 1997. For all data, better than one percent accuracy in transmission as a function of energy was maintained over the entire filter area. The resulting transmission maps reveal spatial non-uniformities in the filters of about 0.5% to 2%. These transmission maps provide the finest spatial calibration ever achieved on such filters.