Summary and Conclusions

The X-ray transmission of the ACIS-I and ACIS-S flight-like filters was measured at NSLS in the energy range between 0.26 and 3 keV. We have presented a means of constructing an X-ray transmission function for the ACIS Imager and Spectrometer filters that can be expressed as a function of atomic scattering factors and EXAFS effects. The transmission function follows the data to better than 0.5% in non-XAFS regions and better than 1% in the C-K and Al-K EXAFS regions. For energies within several eV of the absorption edges, larger deviations of a few percent are observed. These can be explained as a combination of the limits in the resolving power of the monochromator used in our measurements, the uncertainties in the atomic scattering factors near absorption edges and to the presence of resonance lines and narrow near edge X-ray absorption structure within several eV of the absorption edges. A detailed investigation of the spatial uniformity of the ACIS filter performed by Townsley et al. 1996 show the ACIS filter X-ray transmission uniformity to be better than 2% over the entire filter area, down to spatial scales of $\sim$1 mm. Finally as our future scientific goals for X-ray observations become even more demanding subtle effects such as temperature dependence of EXAFS will need to be considered. We presented a simple model to describe the change in the X-ray transmission of a filter with temperature and estimated a change of less than 0.5% in the amplitude of the Al-K EXAFS for the ACIS filters for temperatures ranging from 20 C to -60 C.