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Under normal operation, each ACIS CCD is read out in parallel through 4 output nodes that split the CCD into quadrants. Each node has its own set of electronics, thus the gain and noise varies from quadrant to quadrant.
The output nodes are commonly referred to as nodes 0 thru 3, and the quadrants are commonly named A thru D. Thus someone talking about node 0, or `c0', and quadrant A are talking about the same data. [Footnote: It is actually possible to read the CCDs out thru only 2 output nodes. Then the idea of quadrants and nodes becomes more confusing.]