Temperature and velocity-distribution remote-sensing of faint diffuse sources such as the interplanetary medium (IPM), comets and planetary atmospheres, is an instrumental challenge that becomes more pronounced in the ultraviolet. All reflective Spatial-Heterodyne Spectrometers (SHS), an emerging new class of instruments that combines both high étendue and high resolving power (greater than 105), are ideally suited to these types of observations. Their all-reflective configuration and their self-compensating monolithic design enable them to operate under the tight tolerances of the ultraviolet and to survive the rigors of space launch. An in-development sounding-rocket experiment, the Hydrogen Polarimetric Explorer (HYPE), will merge an all-reflective SHS with a half-wave Brewster reflection polarimeter to obtain the first interferometric polarimetry of an ultraviolet emission line source. Its initial flight will target the IPM at the hydrogen Lyman-alpha transition (121.6nm). HYPE's novel optical configuration also combines several improvements in reflective SHS design, including true zero-path interferometry, no aliasing, and one-dimensional imaging. The optical layout and performance of the HYPE prototype will be described along with simulation results from ray-tracing computations.