Possible physical causes of the trends removed by SYSREM. The gray regions and dashed vertical lines are the same as for Figure 4. Note the sudden drop in S/N as the seeing begins to deteriorate. It recovers as the exposure time is increased but continues to degrade as the seeing worsens, only recovering when the seeing stabilizes. This trend is coincident with a rapid change in the wavefront coherence time, ﹩{\tau }_{0}﹩. The seeing and ﹩\tau 0﹩ were acquired from the VLT Astronomical Site Monitor (VLT-ASM). The flux is the raw value recorded by the adaptive optics (AO) sensor, and the temperature is the telescope ambient temperature.