READ vs. CHIP coordinates

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.

• CHIP coordinates are physical pixel units on the CCD.

• READ coordinates are virtual pixel units that describe how the pixels were read from the CCD.

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.

  

Figure 3.2: CHIP vs. READ coordinates

The transformation from READ to CHIP coordinates includes the `flipping' of quadrants B and D and the removal of overclocks as described below.