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A formation flying interferometer suffers a unique stray light problem: it is the only space astronomical instrument which looks at starlight in the presence of a bright object (the sunlit collector spacecraft) which is intentionally placed in the field of view. The combiner instrument is a collection of astronomical telescopes looking at stellar targets relayed by the collectors; but other parts of the collector spacecraft can scatter or emit substantial fluxes. Screening the glare while collecting the starlight is the challenge. The TPF formation-flying interferometer is especially vulnerable because the planets it seeks are so faint. We present a preliminary estimate of stray light from various sources appearing in the interferometer science detector. We assumed a pinhole field stop would be employed, but that two mirrors preceding that pinhole in the combiner optical system would scatter off-axis light into the pinhole. Sources include scattering of direct sunlight and thermal emission, both from the multi-layer thermal shades which permit passive cooling of the instruments to about 40K. We find that the estimated stray light fluxes are of order 10 4 times the planet flux. We conclude that the optics at the combiner entrance must be blocked from any direct view of the thermal shades.

There is a growing consensus that a substantial fraction of the matter in the universe, especially what we think of as normal baryonic matter, exists in a tenuous, hot filamentary intergalactic medium often referred to as the Cosmic Web. Improving our understanding of the web has been a high priority scientific goal in NASA's planning and roadmapping activities. NASA recently supported an Origins Probe study that explored the observable phenomenology of the web in detail and developed concepts for the instrumentation and mission. The Baryonic Structure Probe operates in the ultraviolet spectral region, using primarily O VI (λλ 1032, 1038Å) and HI Ly α (λ 1216 Å) as tracers of the web. A productive investigation requires both moderate resolution (R = λ/∆λ ~ 30000) absorption line spectroscopy using faint background quasars as continuum sources, and imaging of the diffuse filaments in emission lines of the same ions. Spectroscopic sensitivity to quasars as faint as V ~ 19 will probe a large number of sight lines to derive physical diagnostics over the redshift range 0 < z < 1. Spectral imaging with a wide field of view and sensitivity to a redshift range 0 < z < 0.3 will map the filaments in a large volume of the universe after the web had evolved to near its modern structure. This paper summarizes the scientific goals, identifies the measurement requirements derived from them, and describes the instrument concepts and overall mission architecture developed by the BSP study team.

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