Overview of structures visible in the detector flat-field. Left: master flat-field generated by averaging hundreds of individual flat-field frames. Right: one component of an unranked empirical principal component decomposition of the temporal variance of the flat-fields into five basis vectors. This component highlights minute details of the flat-field variability. Details that are normally not clearly visible become apparent due to the changes in illumination by the internal calibration lamp affecting diffraction patterns over time. This includes the fringe-like horizontal stripes, as well as dust diffraction by various optical surfaces.