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Accurate Ground-based Astrometry of Naked-eye Stars: The United States Naval Observatory Bright-Star Astrometric Database

We present the United States Naval Observatory (USNO) Bright-Star Astrometric Database (UBAD), a current-epoch high-accuracy astrometric catalog. The catalog consists of 364 bright northern hemisphere stars, including all but five such stars with either $V < 3.5$ or with $I < 3.2$ and $V < 6$, as well as a large fraction of slightly fainter stars; 36 of the brightest catalog stars are not included in Gaia Early Data Release 3 (EDR3). Observations were conducted with the USNO, Flagstaff Station, Kaj Strand 61-inch Astrometric Reflector. Target stars were imaged through a small 12.5-magnitude neutral-density spot, while the remainder of the stars in the field of view were unattenuated. This allowed for unsaturated images of the bright target stars to be calibrated directly against much fainter reference stars from Gaia EDR3. The median position errors are 1.9 mas in both right ascension and declination at the catalog epoch of 2017.0, with 90% of catalog stars having errors less than 2.6 mas; systematic errors are 1 -- 3 mas. Combining UBAD observations with Hipparcos-2 positions yields proper motions with median errors of 0.045 and 0.049 mas year$^{-1}$ in right ascension and declination, respectively, with 90% of stars having errors less than 0.1 mas year$^{-1}$; systematic errors are about 0.1 mas year$^{-1}$. Single-frame accuracy for positions of the target stars is typically 5 -- 6 mas. Gaia EDR3 astrometry for these bright stars, which are heavily saturated in the Gaia observations, is validated over the magnitude range $2 \lesssim G \lesssim 6$.