A strange phenomenon can be observed in the JB offset plot: m1-m2 is not constant with m1, it decreases with increasing m1. If the y-axis had been extended, this would be even more clear. The sky determination in both images was checked and was found to be OK. The phenomenon may either be related to fat zero at low levels (both images have low background levels), or related to the 2 images being stacked from several (2 or 4) shorter exposures. In any case, the bright stars are believed to not be significantly influenced by this.
Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997
Bo Milvang-Jensen (email@example.com)