Absolute calibration of ACIS X-ray CCDs using calculable, undispersed synchrotron radiation

We have performed a series of measurements with the Physikalisch-Technische Bundesanstalt beamline of the electron storage ring BESSY I which provide the basis for the absolute calibration of the Advanced CCD Imaging Spectrometer (ACIS). ACIS is a prime focal plane instrument aboard the recently-launched Chandra X-ray Observatory. We have achieved an absolute detection efficiency knowledge accurate to better than 5% over the 0.3 - 4 keV band. We describe our measurement and analysis techniques, including our detector response modeling and pileup corrections. We summarize a variety of external and internal consistency checks which provide the basis for our error estimates. We discuss the factors limiting the accuracy of our measurements.