Long-term stability of image quality, astrometry, and photometry. All panels show image-level quantities as a function of exposure index, which follows the chronological observing sequence. (a) Median FWHM for each exposure, tracing the temporal behavior of the seeing; vertical dashed lines mark the boundaries between observing nights. (b) External astrometric rms from SCAMP, both for all matched sources and for a high-S/N subsample, together with their median values, quantifying astrometric solution stability. (c) The magnitude where the median formal uncertainty equals 0.01 mag (≈1% precision) and its evolution with time. (d) Photometric zero-point per exposure, with its median value overplotted, illustrating temporal stability and slow drifts in photometric calibration. In all panels, the horizontal dashed line denotes the median value over the full sample of exposures.