Color−magnitude diagram for sources in the field of NGC 55. All photometrically measured sources are marked with a red dot, while those residing within the isophotal limits of the galaxy are represented by green dots. Overplotted are lines denoting the limiting magnitude of 18.0 and a [3.6]−[4.5] color of 0. On the left are ellipses corresponding to the average uncertainty for all points in every one magnitude bin.