Inconsistent illumination across the slit
TSMC also seemed to know about it, at one point, from their patent US9091930 https://patents.google.com/patent/US9091930 where they considered rotating patterns along the slit (Figures 3B, 4).
This pupil rotation is also disclosed in Zeiss's patent US6859515 https://patents.google.com/patent/US6859515B2/ see Figure 5
TSMC also seemed to know about it, at one point, from their patent US9091930 https://patents.google.com/patent/US9091930 where they considered rotating patterns along the slit (Figures 3B, 4).
This pupil rotation is also disclosed in Zeiss's patent US6859515 https://patents.google.com/patent/US6859515B2/ see Figure 5