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2. A Description of the Phenomenon

• Each series of ACCUM observations starts with approximately the same counts as the previous series of the same kind, and then declines steadily. The drop follows roughly an exponential curve.

• The percentage decline is approximately the same for both star and wavecal, yet the total count rates differ by more than a factor of 100.

• The wavecals, because of their high signal-to-noise, clearly show a consistent structure in the decline curves: An initial rise in counts, then a rapid decline, then a more gradual decline.

• The wavecals decline evenly in counts (there is no wavelength dependence); i.e., there is no apparent drop in one end of the spectrum relative to the other.