3.1.3.5 Sub-Array Readout and Staggered Readout

  Another way to reduce the frametime, and hence reduce photon pileup, is to use fewer rows of the CCD. There are two ways to do this, called Sub-Array Readout and Staggered Readout.

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:  

$$t~=~41 \times (m-1) + n \times 2.85 + ( m \times q ) \times 0.04~{\rm msec}$$ (4)

where n is the number of rows selected for analysis, m are the number of CCDs turned on and q are the numbers of rows to be skipped between the output node and the start of the sub-array. Integration time is controlled by an entry in the command parameter block which has a resolution of 0.1 seconds, so the time given in Equation 3.2 is rounded up to the next 0.1 second. The minimum time (also given by power and thermal considerations) is a 100 row region located at the output node, which requires roughly 0.5 second. For a 100 row region located at the top of the CCD (farthest from the output node), the minimum 100 row sub-array frametime is 0.9 second.

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.

  

Exposure Exposure

Size (rows) Time (msec)

18 59.13

38 115.33

54 160.29

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.