In experiments with intense x‐ray pulses, perturbations of the signal in traversing a diagnostic line‐of‐sight channel connecting source and detector can be large, requiring detailed computations to infer true characteristics of the signal. These perturbations fall in the most difficult class, being highly nonlinear (especially with respect to frequency) and time dependent. They cause the transmission of collimators and vacuum windows to change continuously as these apertures heat, ionize, and expand under intense irradiation; similarly the fluorescence efficiency of a scattering foil, the transmission of a filter, the reflectivity of a multilayer mirror, etc., can change significantly over the x‐ray pulse duration. We discuss the problem of modeling x‐ray transport through the ablation plasmas created by the intense signal, focusing on the questions: What are the sources of systematic error in our calculations that limit our ability to correct for aperture closure effects and how can the size of perturbations be minimized by careful design of the line‐of‐sight channel? This work was performed under the auspices of the U. S. DOE by the LLNL under Contract No. W‐7405‐ENG‐48.