A Thomson scattering experiment is under construction for the central cell of the Tara tandem mirror. This experiment has been designed to measure electron temperature and density at densities as low as 3×1011 cm−3. The measurement is made at a single time point per shot using a Holobeam 10‐J Q‐switched ruby laser, folded back for two passes through the scattering volume. Detection is at 90° with f/5 optics. The optical table is mounted on linear bearings with a 10‐in. stroke, so that the radius of the scattering volume may be changed from shot to shot. Novel features of the experiment include a high‐throughput (>25%) commercial triple‐grating polychrometer using holographic gratings, and a directly coupled ten‐channel microchannel plate photodetector array.