Towards sub-milliarcsecond astrometric precision using seeing-limited imaging

TL;DR

This work evaluates the feasibility of sub-milliarcsecond astrometry using seeing-limited ground-based data by leveraging the Korea Microlensing Telescope Network (KMTNet). It introduces a Gaia-like AGIS-inspired reduction pipeline with detrending to control systematics (chromatic, annual, intra-pixel, SYSREM) and demonstrates performance on KMTNet fields BLG17K0103 and BLG15M0306. The results show per-epoch precision of ~5 mas for bright sources, with time-binned gains to ~2 mas per coordinate, and a bootstrap-based proper-motion precision near 0.1 mas/yr; Gaia DR3 comparisons yield ~0.3 mas/yr, highlighting both opportunities and frame-related systematics. The study confirms the viability of ground-based astrometric microlensing to detect isolated stellar-mass black holes and underscores the value of cross-telescope combinations for improved precision. Overall, the methodology provides a practical path to high-precision, seeing-limited astrometry in crowded bulge fields, complementing space-based surveys like Roman/WFIRST.

Abstract

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.

Figures (17)

• Figure 1: Source density for KMTNet BLG17K0103 field. The density is calculated based on the KMTNet catalogue, and the units are the number of sources per PSF's FWHM area. For the calculation, we use a typical 1D FWHM of 3 pix. We present an example of the $200\times200$ pixels image cutout of the BLG17K0103 in the right-bottom corner.
• Figure 2: Astrometric residuals as a function of the DCR-leading terms in the x-axis. Left panels show the residuals without DCR correction, while right panels show the residuals after applying the correction. Error bars represent the mean and standard error of the mean (i.e., standard deviation divided by the square root of the number of measurements) in each bin of $V-I$ color and corresponding DCR term. Only the top 33% of sources, based on 2D single-epoch precision ($\leq 10$ mas), are included.
• Figure 3: Same as Figure \ref{['fig:chromatic_X_axis_17K0103']}, but for the y-axis.
• Figure 4: Astrometric residuals as a function of the annual phase for both axes, before (left) and after (right) detrending. Residuals are binned in intervals of 0.05 in phase (fraction of year), and colored by $V-I$ color bins as indicated in the legend. Each point represents the median residual within a bin, and the error bars show the standard error. We use the top 33% best sources, with 2D single-epoch precision of $\leq 10\,\mathrm{mas}$.
• Figure 5: Colour–magnitude diagram (CMD) based on Gaia DR3 parameters for a 300$\times$300 pixel cutout of the KMTNet BLG17K0103 field. Colours indicate Gaia DR3 parallax values, with hollow triangles denoting sources with negative parallax.