Towards sub-milliarcsecond astrometric precision using seeing-limited imaging

The Earth's atmospheric turbulence degrades the precision of ground-based astrometry. Here, we discuss these limitations and propose that, with proper treatment of systematics and by leveraging the many epochs available from the Korean Microlensing Telescope Network (KMTNet), seeing-limited observations can reach sub-milliarcsecond precision. Such observations may be instrumental for the detection of Galactic black holes via microlensing. We present our methodology and pipeline for precise astrometric measurements using seeing-limited observations. The method is a variant of Gaia's Astrometric Global Iterative Solution (AGIS) that includes several detrending steps. Tests on 6,500 images of the same field, obtained by KMTNet with typical seeing condition of 1 arcsecond and pixel scale of 0.4 arcsecond, suggest that we can achieve, at the bright end (mag<17), per-epoch relative astrometric precision of ~5 and relative proper motion precision of 0.1-0.2 mas/yr over a baseline of approximately five years, using data from the Cerro Tololo Inter-American Observatory (CTIO) site. Time binning on 5--20 day cadences improves the bright-source precision to ~2 mas per coordinate on astrometric microlensing-relevant timescales. The precision is estimated using bootstrap simulations and further validated by comparing results from two independent KMTNet telescopes.