X-ray CCDs developed at MIT Lincoln Laboratories for the AXAF CCD Imaging Spectrometer (ACIS) have been calibrated in the 0.25 - 1.5 keV spectral range using an erect-field grating spectrometer and an electron impact x-ray source in our laboratory. A combination of the spectrometer with an x- ray CCD on high resistivity substrate allows efficient order separation, and measurement of the CCD energy scale and spectral response function as essentially continuous functions of energy over the spectrometer passband. The CCD energy scale is found to be linear to approximately 0.5% in this spectral band. Relative variations in the detection efficiency of the CCD can also be studied with this system. A near edge structure is detected in the CCD response around the oxygen K absorption edge. Comparison of these results with the data acquired with a synchrotron radiation source and monochromator confirms that these structures are features of the detector response. The edge structure allows us to measure the absolute dispersion relation of the spectrometer (and hence the zero-point of the CCD energy scale) with a precision better than 1 eV. We also detect considerable structure in characteristic nitrogen and oxygen emission lines from the electron impact source. We discuss likely mechanisms for the production of this structure.
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