First off, I don't work for Litton (NGEOS) or ITT.
ABC controls MCP voltage, i.e. gain.
BSP controls PC voltage, i.e. light to electron conversion efficiency.
I have read SPIE paper written by NGEOS and it explained exactly how many volts, etc, but I counldn't locate it. The control loops are closed loop circuits. As for which one works first, I can't really tell you.
ABC lowers the gain if bright light hits it - this is mainly for display purpose from myunderstanding.
BSP on the other hand, lowers the light to electron efficiency. I'm more than sure that there's a certain voltage level that PC works at its optimum, which will generate highest performance.
Obviously, for the highest performance, PC must be at its peak operating quantum efficiency and the gain in MCP must be at its optimum.
If you lower the PC efficiency, the gain must be turned up - just as you lower the CD plaer volume output, you have to turn up the volume in the amplifier. All the components have a noise floor. In PC, it's the dark current (current that PC still gives out while there's absolutely no light).
It can get further in depth, but it just gets more complicated and there'll be more questions than answers. This is why optical system design is done with modeling - working form the front to back, i.e. what do you want to see and how far, then work backward to f/#, aperture size, effecrtive focal langth, coatings, etc.