Kutsaev, S.V. Goncharik, V. Murokh, A.Y. Ruelas, M. To, H.L. JACoW Geneva, Switzerland 978-3-95450-177-9 10.18429/JACoW-IBIC2016-WEPG57 English 783-785 WEPG57 detector radiation electron diagnostics alignment Contribution to a conference proceedings 2017 2017-02 http://dx.doi.org/10.18429/JACoW-IBIC2016-WEPG57 http://jacow.org/ibic2016/papers/wepg57.pdf We present a new diagnostics instrument designed to measure bunch length in RF particle accelerators. Typically, scanning-type Michelson or Martin-Puplett interferometers are used to measure the coherent radiation from a short bunch. However, they require averaging over several shots over several minutes, thus being able to report only the average bunch length. We propose to measure the emitted coherent spectrum of a short bunch emission that contains the same spectral information as the bunch shape by means of single-shot spectrometry. In this paper we present design considerations, and first experimental results obtained at FACET for the instrument that allows shot-to-shot measurement of the emitted spectrum.