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PSTN-052.tex

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 \section{Introduction}
 
-The Rubin Science Requirements Document (SRD) stimulated that Rubin should survey 18,000 square degrees with a median of 825 visits (spread over the $ugrizy$\ filters) after 10 years. This works out to one observation every 4.4 days (or one observation every 2.6 days if we have a 7-month observing season). For many transient phenomenon, including SNe Ia, this cadence is not ideal and something slightly higher (e.g., one observation every 2 days) would be preferred. 
+The Rubin Science Requirements Document (SRD) stipulated that Rubin should survey 18,000 square degrees with a median of 825 visits (spread over the $ugrizy$\ filters) after 10 years. This works out to one observation every 4.4 days (or one observation every 2.6 days if we have a 7-month observing season). For many transient phenomenon, including SNe Ia, this cadence is not ideal and something slightly higher (e.g., one observation every 2 days) would be preferred. 
 
 There is no free lunch. Once we have defined a survey footprint and telescope efficiency, the mean observing cadence is highly constrained. The additional preference to take observations near the meridian further constrains the possible cadence and season length. Going to higher cadence at one time demands that there be a corresponding period of lower cadence to ensure footprint coverage uniformity. Similarly, if an area has a higher cadence, another point at similar RA must also have a lower cadence because the telescope efficiency is fixed.  In Figure~\ref{fig:ideal_foot} we show the ideal cumulative number of observations for different observing strategies. Figure~\ref{fig:ideal_foot} also shows that {\bf{rolling and non-rolling cadences have the same mean and median cadence}}. When comparing simulations, it is important not to make approximations that average an observing cadence, as the effects of rolling will be lost.