Current laboratory performance of starlight suppression systems and potential pathways to desired Habitable Worlds Observatory exoplanet science capabilities

We summarize the current best polychromatic (<FOR VERIFICATION>10% to 20% bandwidth) contrast performance demonstrated in the laboratory by different starlight suppression approaches and systems designed to directly characterize exoplanets around nearby stars. We present results obtained by internal coronagraph and external starshade experimental testbeds using entrance apertures equivalent to off-axis or on-axis telescopes, either monolithic or segmented. For a given angular separation and spectral bandwidth, the performance of each starlight suppression system is characterized by the values of "raw"contrast (before image processing), off-axis (exoplanet) core throughput, and post-calibration contrast (the final 1-sigma detection limit of off-axis point sources, after image processing). Together, the first two parameters set the minimum exposure time required for observations of exoplanets at a given signal-to-noise, i.e., assuming perfect subtraction of background residuals down to the photon noise limit. In practice, residual starlight speckle fluctuations during the exposure will not be perfectly estimated nor subtracted, resulting in a finite post-calibrated contrast and exoplanet detection limit whatever the exposure time. To place the current laboratory results in the perspective of the future Habitable Worlds Observatory (HWO) mission, we simulate visible observations of a fiducial Earth/Sun twin system at 12 pc, assuming a 6 m (inscribed diameter) collecting aperture and a realistic end-to-end optical throughput. The exposure times required for broadband exo-Earth detection (20% bandwidth around <FOR VERIFICATION>λ=0.55 μm) and visible spectroscopic observations (<FOR VERIFICATION>R=70) are then computed assuming various levels of starlight suppression performance, including the values currently demonstrated in the laboratory. Using spectroscopic exposure time as a simple metric, our results point to key starlight suppression system design performance improvements and trades to be conducted in support of HWO's exoplanet science capabilities. These trades may be explored via numerical studies, lab experiments, and high-contrast space-based observations and demonstrations.