Next: 22.214.171.124 `Smear' Exposure Times Up: 3.1.3 On-board Data Processing Previous: 126.96.36.199 Alternating Exposure Times
In Sub-Array Readout only a reduced number of rows of pixels are processed and pixel data from the rest of the chip discarded. The time required to readout a CCD frame in this mode depends on the number of rows of pixel data within the window selected and the location of the rows on the CCD. There is a fixed minimum overhead of 41 msec required to clock out the CCD at maximum speed. In addition the farther the region is from the output node, the more time must be allowed to move the charge to the output node for processing. Finally the frame transfers must be staggered to reduce the peak power requirements, so the frame time for Sub-Array Readout is:
If these frametimes are too long, then Staggered Readout is possible. In this case the integration region of the chip is not completely transfered into the framestore array. Instead only a smaller number of rows are shifted, leaving the previous imaging on the integration region but shifted to a different location. If the target field consists of a bright source with few other sources of X-rays, then the CCD chip will fill with multiple images of the target. When the frame is eventually completely readout the image will look like a multiply exposed piece of photographic film, with a series of staggered images vertically displaced from each other.
|Size (rows)||Time (msec)|
In this mode with only one CCD turned on, the exposure times in milliseconds are given in Table 3.3. Note that Stagger Mode was used during XRCF calibrations, but it required a software patch to execute. Its availability during flight operations would require the patch.