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\subsection{Photometric Accuracy}  \textbf{Relative Photometric Calibration} The success of ZTF hinges on the ability of the survey to provide an accurate and precise relative photometric calibration.  The dual grid of ZTF fields recommended above will provide significant overlap between every ZTF observation, which will, in turn, allow a precise calibration of the relative photometric system. \citet{Padmanabhan_2008} demonstrated that it was possible to use overlapping observations from SDSS to obtain a survey-wide relative photometric calibration of $\sim$10 mmag, while \citet{Schlafly_2012} demonstrated that the same algorithm could be applied to PS1 observations to obtain a relative precision $<$ 10 mmag.   \textit{Specification}: for bright sources that are not limited by photon statistics, the rms of the unresolved source magnitude distribution about the source mean value will not exceed PA1 mmag. No more than PF1 \% of the measurements will deviate by more than PA2 mmag from the mean. (Table~\ref{tbl:relative_phot}).  \begin{table}  
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\end{tabular}   \caption{\label{tbl:relative_phot} The specifications for the relative photometric accuracy in the $g$ and $r$ bands. If the previously identified stretch goal is realized and an $i$ filter exists for the system, then the PA1 and PA2 in this filter may be 50\% worse.}  \end{table}  This requirement is driven by all of the major science goals for ZTF, while the stretch goals are defined to specifically improve the ability of the survey to characterize variable sources. sources and search for low-amplitude events such as planetary transits.  \textbf{Absolute Photometric Calibration} Many of the ``bread and butter'' science goals of ZTF require a precise and accurate absolute photometric calibration (e.g., cosmography with Type Ia SNe, mapping Galactic structure with RRL). Recently, PS1 has demonstrated the ability to produce an absolute calibration, relative to the Sloan Digital Sky Survey and \textit{Hubble Space Telescope} Calspec stars of $\sim$30 mmag \citep{Tonry_2012}. PTF achieved $20\,\textrm{mmag}$ scatter compared with the SDSS \citep{Ofek_2012}. However, since PTF observed mostly in a single filter, one needs to know the star color in order to achieve the this accuracy.  The first requirement is that ZTF will observe the sky in at least two bands.