Stacking is how random noise is beaten. Averaging N frames improves signal-to-noise by √N (4 frames → 2×, 100 frames → 10×). Frames are first registered (aligned on stars), then combined. Sigma clipping rejects pixels that deviate too far from the median — removing satellite trails, cosmic rays, and plane lights without harming real signal. Dithering (shifting the mount a few pixels between frames) decorrelates fixed patterns and walking noise so they average away during integration. Total integration time is the real lever: more, longer, well-dithered subs make cleaner images.