Impact of 3D effects on the characteristics of a multi-monochromatic x-ray imager.

The multi-monochromatic x-ray imager instrument records arrays of time-gated spectrally resolved images of inertial confinement fusion targets. In the past, the image characteristics and spectral resolution of the instrument were estimated using a paraxial approximation and ignoring the substrate thickness of the pinhole array (PHA). In this work, we remove these two approximations to derive new analytic approximations and test them using a 3D ray tracing model. We show that variations in spatial resolution and transmission through the PHA are realized once the two approximations are removed simultaneously. A combination of the variation in pinhole transmission and solid angle gives rise to an image brightness modulation within the detector area. We also find a limit on the spectral resolution caused by an effective 2D dispersion from the multi-layered mirror and a spectral broadening related to spatial resolution. These findings enable better data processing and synthetic data for testing, and give further knowledge of the instrument's characteristics and limitations. (C) 2019 Optical Society of America