0 Assigned ID: 410 ----- Attempting Submission 1 (Fri Apr 07 06:08:13 GMT 2017) ----- HST Phase I Proposal 410 (gc_hst25_phaseI.apt) successfully submitted. Receipt: # 410-1 ----- Attempting Submission 2 (Fri Apr 07 17:49:47 GMT 2017) ----- HST Phase I Proposal 410 (gc_hst25_phaseI.apt) successfully submitted. Receipt: # 410-2 ----- Attempting Submission 3 (Fri Apr 07 20:58:23 GMT 2017) ----- 0 ----- Attempting Submission 1 (Thu Jul 20 18:15:05 GMT 2017) ----- ----- Attempting Submission 2 (Wed Aug 08 21:04:56 GMT 2018) ----- The short-period star S0-2 will reach its closest approach to the supermassive black hole at the Galactic center in 2018 at a separation of < 120 AU. This event will offer the first of a series of tests of General Relativity with stellar orbits around a supermassive black hole, an unexplored regime for tests of gravity. While S0-2 has been monitored for over 20 years with high angular resolution measurements from the ground, the astrometric reference frame is currently not stable enough for GR tests. The current reference frame is constructed using observations of 7 radio masers with narrow-field adaptive optics imaging from the ground. While this method provides a reference frame that is stable enough to measure the Keplerian orbits, it does not meet the more stringent requirements needed to measure post-Newtonian effects such as the precession of the periapse of the orbit. The small number of reference stars and the systematic errors associated with using large mosaics to observe these stars limit the accuracy of this method. We propose to construct a new reference frame for tests of gravity with WFC3-IR observations in Cycles 25, 26, & 27. When combined with absolute positions and proper motions from GAIA, these observations will provide the required precision in the reference frame. The WFC3-IR field of view has over 1000 times greater areal coverage than the narrow-field AO observations, providing 3 times the number of absolute reference sources from GAIA. The proposed observations build on a legacy of HST, GAIA, and adaptive optics data to open an era of gravitational science with orbits at the Galactic center. This is an astrometric program to measure precise astrometry for a sample of stars in common with GAIA at the Galactic center in order to build a stable reference frame for the measurements of orbits around the supermassive black hole at the Galactic center. The stability of the reference frame is driven by the goal of measuring the effect of General Relativity on the orbit of the star S0-2. As such, this program requires very high astrometric precision and minimal systematic errors. We have designed the Phase II observations to achieve these goals. We request that observations in Cycle 25 to have an ORIENT=270 deg, the same as GO-13770, in order to optimize the astrometric performance and minize the effect of residual optical distortions. We request that the observations be made between 2018-02-01 and 2018-05-31 in order to be close in time to GAIA observations of this region. This time range will also allow us to observe near the closest approach of the star S0-2 to the supermassive black hole in 2018. We request observations in the F153M filter with a spiral dither pattern for astrometry. We also plan to observe with the F127M filter to check against the astrometry at F153M, since the stellar crowding is much higher at F153M. Since the GAIA sources are very blue compared to the background Galactic center sources, we will be able to use the F127M astrometry instead of F153M for confused sources at F153M. Stellar Populations Massive Black Holes And Their Host Galaxies 6 false false false false false false false SUPPORTED STELLAR CLUSTER NUCLEUS NO This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/IR Imaging No No F127M WFC3/IR Imaging No No F127M WFC3/IR Imaging No No F127M POS-TARG DITHER 0.42 LINE false false 15 11.0 SPIRAL 6 false POS-TARG DITHER 0.572 LINE 0.365 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 7 false /private/var/folders/hp/k95gxtfs4dqdcchs9nf9dpv40000gp/T/OPVisit6351677221133425378.xml 1533755004333 The scheduling data was generated by CASM 26.0. CASM was run in Phase II mode. 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