had a range of 1–2, with a median value of $1\farcs24$. For the image sets with some non-photometric data, we calculate the “effective” exposure time, which is the equivalent exposure time that would be necessary during photometric conditions to reach the same flux levels we measure in our images. This information is summarized in Table \[table:mosaicobs\].

Data Reduction
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The CTIO Mosaic camera consists of eight $2048\times4096$ CCDs arranged in a configuration that produces a $8196\times8196$ pixel focal plane. We reduced the data using the IRAF[^1] tasks provided in the MSCRED package (Valdes 1998). These tools were specially designed for the reduction of mosaic CCD data. We applied standard techniques for CCD reduction, including overscan and bias subtraction, and flat-fielding. We took and used both dome, twilight, and night sky flats. We applied a sinc interpolation for astrometric regridding. The varying spatial scale in the field of view, where pixels farther from the image center subtend more sky, means that flat-fielding corrections overestimate the sensitivity of pixels at the field’s edge. The regridding process compensates for this effect and redistributes the flux properly.

Once images were regridded, we found it necessary to introduce another scaling correction. Due to either gain 