The results of the Monte-Carlo ray-tracing simulations for a Johann-type Bragg spectrometer with spherically curved-crystal designecl Lo detect the X-rays from a fast-moving source are reported. These calculations were performed to optimize the X-ray spectrometer to be used at the gas-target installed at ion storage ring for high-resolution X-ray experiments. In particular, the two-dimensional distributions of detected photons were studied using the Monte-Carlo method both for the stationary ancl moving X-ray sources, taking into account a detailed description of X-ray source ancl X-ray cliffraction on the crystal as well as a role of the Doppler effect for in-beam experiments. The origin of the asymmetry of observecl X-ray profiles was discussed in detail ancl the procedure to derive a precise (sub-eV) X-ray transition energy for such asymmetric profiles was proposed. The results are important for the investigations of 1s2p(3)P(2) -> 1s2s(3)S(1) intrashell transition in excited He-like uranium ions in in-beam X-ray experiments. (C) 2014 Elsevier B.V. All rights reserved.