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A Johann focusing elliptical spectrograph has been developed for the measurement of high-resolution x-ray spectra from a spatially extended source. The instrument was designed for the study of high-density, high-temperature plasmas produced by z-pinch implosion or ion-beam bombardment on pulsed-power accelerators. We have constructed and tested this instrument, and have demonstrated an improvement in resolution over what we obtain with a standard circular detector when viewing an extended source. Analytic results for the second-order source broadening due to a finite source size have been obtained and verified by ray tracing. Also given is a simple parametric equation for the Johann crossover curve in terms of either the ellipse sweep angle λ or the Bragg angle θB.
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Hammel et al. (1990) studied this question.
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