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Ten X-ray CCDs will be placed in the focal plane of the AXAF Imaging Spectrometer (ACIS). X-ray CCD technology matured now to the level where spectral resolution is close to the theoretical limits even for the big arrays. Each device has an image section of 1024 x 1024 pixels and is manufactured on a high resistivity (5000 ) silicon. Detailed description of the ACIS CCD can be found elsewhere[1]. Both frontside illuminated CCDs and backside illuminated devices will be utilized in the instrument focal plane. To perform a calibration of these devices in a low energy band from 0.25 to 1.5 keV, each device was put into the focal plane of the soft X-ray High Resolution Erect-Field Spectrometer (HIREFS 2) combined with an electron impact X-ray source.

An X-ray CCD coupled with an X-ray spectrometer offers an extremely powerfull tool for studying the properties of both CCD and an X-ray source used in the measurement. Emission lines seen in the output spectrum reveal wealth of information about the material of the target of the electron impact source, whereas edges in the spectrum and intensity variations tell a lot about CCD response function. Here we present some results of these measurements.