An Empirical Decomposition of the ACIS Optical Blocking Filter

In order to facilitate adjustments, the released model of the OBF transmission was decomposed using IDL into component transmission functions that exactly reproduce the input model.  These components represent estimates of the transmissions of elements in the filter so that they could be adjusted independently when fitting data.

The decomposition proceeded from low energy to high.  The Al-L edge was assigned all opacity below the C-K edge, or, more precisely, below an energy where the detailed absorption structure associated with the edge begins.  The Al-L opacity was extrapolated to higher energies using the data from below C-K as a guide by fitting to a transmission of the form: T = T_0 exp[ - E/E_0)^a ].  A simple exponential curve was used to take out systematic deviations from this relation near the edge energies.  The data above the C-K edge were divided by the extrapolation of this model and then the next edge (N-K) was treated in a similar fashion.  In a series of 4 steps, extrapolation parameters were estimated and used to determine the strength of the next edge.  The Al-L and Al-K tranmissions were derived and maintained separately but are, naturally, physically linked.  The transmission functions are tabulated here.  The IDL code is available.

The decomposition is shown in this figure.