On zero-point drift in astronomical photoelectric photometry

Extract

In variable star work, the frequent observations of comparison stars during a night often clearly indicate the presence of either changes in extinction or changes of zero point of the photometric system, or both effects simultaneously. In a magnitude — air mass plot, the graph is not linear, but is usually a loop. These loops are well known to observers but so far very little has been published about this problem. Young’s (1974) chapters are perhaps the only exception.