Diagnostic tests for the ACA high background fault tree:

A: Digital Electronics: not a likely cause, since the
   digital electronics processing of ACA data is common
   to all images, and the high background only affects 
   some images.

B: While saturation effects can occur in the analog
   electronics common to all 4 CCD quadrants, Ball
   Aerospace has stated that these effects only persist
   for short amounts of time, i.e. the sampling and
   digitization of 1 or 2 unsaturated pixels may be
   affected by the immediately previous sampling and 
   digitization of saturated pixels. Since high background
   events can affect multiple images over several readouts,
   analog electronics saturation is unlikely to be the
   cause.

C: Direct Light: test for Sun, Earth, Moon, Planets in the
   ACA field of view at the time of high background. Of
   the planets, only Venus and perhaps Jupiter would be 
   capable of affecting the background measurement in
   several image slots across different quadrants.
   Some satellites might be bright enough to saturate
   the ACA, but would presumably have small angular
   offsets from the Earth.

D: Scattered light: correlate episodes of high background
   against (theta,phi) coordinates relative to the ACA line
   of sight, to try to identify scattering paths. Also 
   correlate episodes of high background against angular 
   sizes (i.e. distances to) Earth and Moon. Also correlate
   episodes of high background against times of firing of
   MUPS thrusters, in case light can be scattered into ACA 
   by MUPS exhaust plumes.