GBTDS F146 per-exposure astrometric precision σ_ast (top panel; solid gray line) and SNR (bottom) as a function of magnitude calculated from the simulations of K. A. McKinnon & R. P. van der Marel (2026). The dashed lines show the scaling relationship in the source-dominated regime (SNR ∝ counts^0.5) and the background-dominated regime (SNR ∝ counts). Centroiding of 1% of a pixel is a commonly assumed astrometric precision (WFIRST Astrometry Working Group et al. 2019; K. A. McKinnon & R. P. van der Marel 2026), corresponding to a floor of 1.1 mas (black curve) for Roman. Centroiding to ∼0.1% of a pixel would be required to take full advantage of the SNR in the source-dominated regime.