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\subsection{Astrometric Accuracy}  The astrometric requirements for ZTF are defined for stars that are not limited by photon statistics. The requirements are specified for both relative and absolute astrometric measurements.   \textbf{Relative astrometric calibration}  \textit{Specification}: The rms of the astrometric distance distribution for stellar pairs with separation of $D$ arcmin (repeatability) will not exceed AMx milliarcsec (median distribution for a large number of sources). No more than AFx\% of the sample will deviate by more than ADx milliarcsec from the median. AMx, AFx, and ADx are specified for D=5, 20, and 200 arcmin for x = 1, 2, and 3, in the same order (Table~\ref{tbl:relative_astrometry}).  \eran{Do these distances actually make sense?}  The required median astrometric  \textbf{Astrometric Calibration} The detection of moving objects is one of the major goals of ZTF, and the determination of their respective orbits requires an accurate calibration of the astrometric frame. For sources with $r \lesssim 20$ mag, SDSS was able to obtain an accuracy of 45 mas rms per coordinate when reduced against the USNO CCD Astrograph Catalog and 75 mas rms when reduced against Tycho-2, with an additional 20-30 mas systematic error in both cases \citep{Pier_2003}. SDSS utilized a larger aperture telescope than ZTF, thus we require ZTF to achieve a 75 mas rms when reduced against \textit{Gaia} for stars with $r \lesssim 19$ mag. \textbf{This should be checked/re-written by someone with further knowledge about astrometry}.