IRAC High Precision Photometry
Finding accurate centroids for photometry is critical because it determines how much flux gets into the aperture. Systematic offsets from the measured centroid to the real centroid cause biases in photometry. Centroiding accuracy appears to be dependent on both measurement technique and geometric effects due to location on the pixel. Measurement techniques include center of light, gaussian fitting, and least asymmetry. Geometric effects of shifting flux into neighboring pixels generates the dependence on pixel location. Testing is ongoing by multiple groups to compare centroiding methods and their biases on photometry. If you are using the gain map provided by the IRAC team, it is important to use the same centroiding method (box_centroider.pro) as was used for the gain map for consistency.