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Due to the physical location of the readout nodes on the CCD, quadrants B and D are read out of the CCD from right to left while quadrants A and C are read out from left to right. This gives rise to two coordinate systems, `READ' and `CHIP' coordinates.
The raw FITS images that the ACIS-2C EGSE outputs are in READ coordinates.
Below is an example illustrating the difference between CHIP and READ coordinates. Suppose charge was physically distributed along a diagonal line from the bottom left edge of quadrant A to the top right of quadrant D, Figure 3.2, it would be read out of the CCD in READ coordinates as shown. Note that the gaps in the right ends of the straight lines are real; they indicate the presence of overclock pixel data in the chip readouts, which don't appear in the CHIP coordinate system.
The transformation from READ to CHIP coordinates includes the `flipping' of quadrants B and D and the removal of overclocks as described below.