Chandra X - Ray CenterMIT Kavli Institute

Some observations are adversely affected by bias-parity errors. To explore the properties of these errors, 16,304 bias-parity error files were obtained from the archive for the interval from the beginning of the mission through July 8, 2010. Figure 1 shows histograms of the total number and the “valid” number (see sec. 1.5) of errors for each file. The results for the valid errors are also listed in Table 1. The results below the horizontal line in the middle of the table are the results for the twelve files that contain errors associated with the “FEP0 problem.” The number of errors, if any, per frame of data is typically very small (Fig. 2 and Table 2). The only bias-parity error files that have more than 10 errors per frame are the twelve files associated with the FEP0 problem. As shown in Figure 3, the number of bias-parity errors varies with time. There is a correlation between the number of errors and the number of pixels on which the errors occur (Fig. 4). Of the 16,304 files, there are only seven where the number of valid errors is not equal to the number of pixels affected by the errors. Six of these seven files are ones associated with FEP0 problem for which the number of errors is comparable to or greater than 262,144, the maximum number of pixels that can be affected by the problem. The seventh file is for OBS ID 7649, where all but one of the 7,439 errors are reported for the pixel (CCD ID, CHIPX, CHIPY) = (8, 786, 356). The distribution of the valid errors in chip coordinates is shown in Figure 5. Inspection of this figure suggests that there is a periodic pattern every 32 pixels in CHIPX. Figures 6 and 7 confirm the existence of a periodic pattern. As shown in Figure 8, there is a similar, but much less pronounced, 32-pixel periodicity in the CHIPX values of the errors associated with the FEP0 problem. A much more prominent feature of the FEP0 errors is that they only occur in columns with odd values of CHIPX (Fig. 9) and in rows with values of CHIPY ≥ 513 (Fig. 8). The remainder of this spec describes how the tool acis build badpix handles bias-parity error files.