4.1 Lab Data

The laboratory calibration of ACIS produces models based on the measurements of:

• quantum efficiency, energy resolution, and system gain and linearity as a function of incident energy and instrument parameters (such as temperature and clock voltages);
• non-X-ray light rejection efficiency, including UV/visual optical blocking filter checks;
• CCD read-noise, charge transfer efficiency, charge collection efficiency, and dark current as a function of energy and CCD parameters;
• geometric layout of CCDs relative to ACIS fiducial marks, including seam locations and sizes;
• limits to six charge transfer clock levels -- high and low on each of: imaging, frame-store, and serial units -- for optimum CTE, noise, and dark current;
• relationship between pre-amplifier bias, reset bias, reset clock levels and read noise, amplifier responsivity, and reset performance (the time to clear the CCD, or dead time);
• verification of read-out modes.

Calibration is done by illuminating the CCDs with low energy X-rays from a monochromator or hard X-rays from a radioactive source. All CCDs have been tested, but some were tested more than others (deeper exposures, for example, which are too time-consuming to perform on all CCDs, but which can be used to characterize them). Absolute efficiency measurements have been done at the BESSY synchrotron radiation facility in Berlin on Lincoln Lab CCDs made in the same way as the flight CCDs. These chips provide transfer standards for assessing the absolute quantum efficiency of all the CCD chips flown in both the imaging and spectroscopic arrays, and the flight backup chips.