CCD x-ray detectors: on-board data processing

We present the results of a comparison of data processing algorithms to be used with space- borne x-ray CCD cameras such as those aboard ASCA, CUBIC and AXAF. The goal is to optimize efficiency and accuracy based upon the capabilities and limitations of the on-board processors. We examine the two main components of processing: determination of the bias (or zero) -level, and event recognition. An algorithm to generate a pixel-by-pixel bias by on-board processing is developed and tested. The on-board bias frame is compared to a bias created from a standard laboratory pixel-by-pixel averaging of dark frames. We show that an accurate pixel-by-pixel bias frame can be created with an on-board algorithm in as few as 15 frames. We show that a bias frame created from that algorithm performs as well as meanframes created in the laboratory. On-board algorithms that handle bias determination and event selection simultaneously are also developed. We show that several types of these algorithms successfully process the CCD data, although the algorithm should be chosen according to the specific capabilities of the processors. The procedures were evaluated by examining event quality and single/split event ratios, and more importantly by the determination of spectral energy resolution (e.g., the FWHM of ⁵⁵Fe). The algorithms were compared and evaluated for laboratory data from several different cameras and types of CCD devices.