intra-CCD zeropoint offsets were determined using the method described in the previous section.

Object Detection and Photometry {#section:detphot}
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We chose the widely used source detection program SExtractor (Bertin 1996) to create catalogs of objects in our three fields. SExtractor is well-designed for photometry of resolved objects on a variable background. This is the situation we face when we look for globular clusters superimposed on the smoothly variable galaxy light. We chose a mesh size (64 pixels or 167) for the background estimation algorithm that was large enough to not affect the photometry of unsaturated objects, but small enough to follow the shell structure of [NGC 5128]{} as well as subtract the wings of bright, saturated stars. The region in the central dust lane was too complex for reliable object detection and we ignored all objects detected there.

Photometry of all program field objects was done using a $3\arcsec$ diameter aperture. We corrected these measurements to $14\arcsec$ diameter apertures using the median growth curve of many bright, isolated stars. Aperture corrections were measure for each image so that seeing variations between images would not be an issue. We also examined the variation of the aperture correction as a function of 