We combine all five of our ditherings into one "stacked" image (per filter). These stacked images comprise our Atlas: the finding charts, if you will, for our photometric catalog.
In combining our dithered images we must first match the intensity scales and sky values for each image. We do this using a script we wrote called, immodestly, philmatch.cl. Secondly, we use the mscred routine mscstack to actually do the combining. Bad pixels are ignored in the combination, leaving us with a cosmetically beautiful rendition. We arbitrarily set the value of any region with no good pixels (such as the core of saturated stars) to a value of 50,000 ADUs. Finally, we shift the images so they line in pixel coordinates, to simplify comparisions and continuum subtraction.