Most of the analysis was performed on data acquired with the ACIS high speed data tap implemented at the XRCF. This mode of data acquisition allowed, in principle, each CCD readout frame to be processed and archived from one of the 10 ACIS devices. The ``tapped'' device was usually illuminated by the focussed X-ray spot. The measurements discussed here were obtained in timed event (TE) mode and staggered readout mode. These modes expose the CCDs to focussed X-rays in subarrays whose sizes depend on the duration of the subarray exposure or frametime (see Table 1). Because the spot size is a strong function of axial location, the subarray size (frametime) was adjusted to accommodate the spot, as is shown to proper scale in Figure 2. Typical subarray sizes and frametimes are given in Table 1.
 |
 |
The staggered readout mode shifts a specified number of rows into the framestore region while exposing the remaining frame to X-rays while the subframe is processed through the detector electronics. This mode results in a 1024 pixel full-frame image with a series of X-ray spots located on each subframe. The number of spots per full frame is 1024 divided by the number of rows per staggered readout frame. In order to analyze the images, the subframes were extracted from the HST full frame images and added to form a single, high signal-to-noise subframe. An example of a staggered readout frame is shown in Figure 3. The short frametime capability of the staggered array mode significantly boosted the photon collection efficiency by providing for high source fluxes with minimal pileup. Its primary disadvantage is that it exposes each subframe to X-rays over the entire readout time of the full CCD frame, resulting in a higher background. For this reason, only the core of the spot was studied using this mode. Staggered readout mode is not an option in-flight. The subarray TE mode CCD frames were likewise added to form a high signal-to-noise images.