By invoking the time, the predicted depth of contamination layer is first calculated from some TBD coefficients, stored in a depth extension. Ad interim we assume a linear growth from a reference point of form:
Depth = Growth . (Time - Reference Time)
The attenuation extension then has linear attenuation coefficients versus
energy appropriate for the witches brew building up on the CCDs. From the
depth value, a transmission factor is thus obtained.