(Proposal ID) S15B-111 (PI) Currie, Thayne (Proposal Title) Constraining the Orbits of HR 8799 bcde with SCExAO (Abstract) SCExAO's now-operational extreme AO system allows vastly improved contrasts/ planet sensitivites over what is possible just with AO-188. We therefore propose deep high-contrast imaging of the benchmark multiple exoplanetary system HR 8799, using AO188/HiCIAO+SCExAO. Thanks to SCExAO's extreme Strehl/ contrasts, we should yield strong detections of all four planets. Coupling exAO with SCExAO's unique astrometric reference grid, we should obtain extremely precise astrometry needed to better constrain the planets' orbits. Our program will increase the astrometric coverage for HR 8799 e by 50% and perhaps solve key issues regarding the system's architecture, in particular whether the planets all have coplanar orbits. The dynamical mass limits derived from these new orbital fits will help calibrate highly uncertain planet cooling models widely used to estimate the masses of comoving substellar companions and determine the demographics of planets at wide separations from imaging surveys.