Probing the Hill Sphere of (486958) 2014 MU69: HST FGS Observations during the 2017 July 17 Stellar Occultation

We observed the 2017 July 17 stellar occultation of HD 168233 by the Kuiper Belt Object (486958) 2014 MU₆₉, the close flyby target of the extended New Horizons mission. Rather than capture a solid body occultation by the KBO itself, our program aimed to constrain the opacity of rings, moons, or other debris in the nearby environment. We used the Hubble Space Telescope Fine Guidance Sensors (HST FGS) instrument in TRANS F583W mode to collect 40 Hz time resolution photometry of the stellar occultation star for two HST orbits during this observation. We present the results of reduction and calibration of the HST FGS photometry, and set upper limits on rings or other dust opacity within the Hill sphere of (486958) 2014 MU₆₉ at distances ranging from ∼20000 km to ∼75000 km from the main body.