Nightstack is a free browser-based tool for stacking a sequence of night-sky photos into a single, cleaner Milky Way image. It works by detecting stars in each frame and matching triangles formed between them against a reference frame, which lets it calculate exactly how much each frame has rotated or shifted relative to the others. The same star-field drift you’d expect from a stationary tripod over a few minutes of shooting. Once aligned, frames are combined using either a straight average or sigma-reject stacking, which discards outlier pixels (a passing satellite, a plane, a stray flash of light) frame by frame before averaging, rather than letting a single bad frame show up as a streak in the final result.

Because the sky and the ground behave completely differently between frames, the sky appears to drift while a locked-off foreground doesn’t move at all. Nightstack separates the two with a sky/ground mask, either detected automatically or painted in manually with a brush. The star-alignment warp only ever applies to the sky side of that mask; the ground is handled separately, either by averaging it across your light frames at their original, unwarped position, or, if you shot a single dedicated foreground exposure at the same framing (common when shooting during blue hour or with a light for fill), by compositing that one frame directly into the ground region instead. An optional dark-frame subtraction step removes fixed sensor noise, and a light-pollution model can flatten horizon glow, deliberately fit only against the darkest portion of the sky, so it doesn’t mistake the Milky Way’s own brightness for pollution and dim it. Exposure, black point, highlights and saturation controls let you finish the tone before exporting a full 16-bit PNG.