Illustration of flat-fielding strategies and their outcomes in the NASIM pipeline. Top panels: ratios of segmented-night master flats (second method) to the single-night master flat (first method), revealing significant sky variations over approximately one hour within a single night. These fluctuations highlight the need for special flat-fielding techniques. Bottom panels: final coadded images using different flat-fielding methods, each covering 0.28 × 0.28 deg² and centred on the galaxy IC 2000 from KEDFS_1_5 tile: Method 1: Single-Night, preserves galaxy outskirts but retains large-scale gradients; Method 2: Segmented-Night, reduces gradients but suffers from persistence artefacts; Method 3: Detections Replacement, attempts to mitigate persistence effects, but introduces background inconsistencies; Method 4: Adaptive Flat, achieves the best balance, effectively reducing both gradients and persistence.