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\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 and search for low-amplitude events such as planetary transits.   \textbf{The spatial uniformity of photometric zeropoints}     \textit{Specification}: The distribution width (rms) of the internal photometric zero-point error (the system stability across the sky) will not exceed PA3 millimag, and no more than PF2\% of the distribution will exceed PA4 millimag (Table~\ref{tbl:uniformity_zp}).   \begin{table}   \begin{tabular}{ rrrr }  Quantity & Design Spec & Minimum Spec & Stretch Goal \\  \hline  PA3 (mmag) &15 & 20 & 10 \\  PF2 (\%) & 5 & 10 & 5 \\  PA4 (mmag) &20 & 25 & 15\\  \hline  \end{tabular}   \caption{\label{tbl:uniformity_zp} The specifications for the spatial uniformity of the photometric zeropoints.}  \end{table}  This requirement is primarily driven by the accuracy of comparing distance modulii to supernovae measured in different parts of the sky. These requirements apply to both the bright and faint end of the survey, and limit the degree of non-linearity in the flux scale.   \textbf{The band-to-band photometric calibration}  \textit{Specification}: The absolute band-to-band zero-point transformations (color zero-points) for main-sequence stars must be known with an accuracy of PA5 millimag (see Table~\ref{tbl:color_zp}).   \begin{table}   \begin{tabular}{ rrrr }  Quantity & Design Spec & Minimum Spec & Stretch Goal \\  \hline  PA5 (mmag) &15 & 20 & 10 \\  \hline  \end{tabular}   \caption{\label{tbl:color_zp} The specifications for the spatial uniformity of the photometric zeropoints.}  \end{table}  \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.  The second requirement is that it will have uniformity of better than 20\,mmag over the entire sky,