One sunlit satellite draws a bright line across a long exposure — and with Starlink and the other megaconstellations overhead, they now arrive in waves. This page does the one thing an almanac can't: it reads the live satellite catalogue and finds the cleanest window to image tonight, for exactly where you're standing.
Astrophotography depends on long exposures — seconds to many minutes — to gather faint light. Anything that moves across the field during that time records as a line. A satellite is bright, fast and perfectly straight, so it lays down a hard, even streak that cuts through nebulae, galaxies and star fields alike. One satellite in one sub-exposure can spoil that frame.
For decades this was an occasional nuisance. The arithmetic has changed. Low Earth orbit is being filled by megaconstellations: SpaceX's Starlink is past 7,000 active spacecraft and licensed for around 42,000; China's Qianfan (Thousand Sails) and Guowang are each filing for tens of thousands more; Amazon's Kuiper and OneWeb add thousands again. More sunlit objects overhead means more streaks per hour — and the AST SpaceMobile BlueBirds, among the largest and brightest satellites ever flown, can blaze brighter than most stars.
THE SCALE — FILED AND FLYING
A satellite is only visible — and only leaves a trail — when two things are true at once: your sky is dark, and the satellite is still catching sunlight. That combination is most common in the band of hours after sunset and before sunrise. You are already in darkness on the ground, but satellites hundreds of kilometres up are high enough to remain lit by a Sun that has set for you. That is the satellite rush hour.
As the night deepens, Earth's shadow extends further out into space and climbs higher across your sky. Satellites that were lit at dusk pass into that shadow and go dark — they are still overhead, but they reflect no sunlight, so they leave no trail. This is the shadow window: the stretch, usually centred on local midnight, when the sky overhead is largely eclipsed and cleanest for deep-sky imaging. Before dawn, the satellites climb back into sunlight and the trails return.
How wide that window is depends on your latitude and the season. Short summer nights at high latitude may never fully clear; long winter nights open a broad, quiet window. The tool above computes it for your exact location tonight, rather than leaving you to guess.
Favour the shadow window for your faintest targets, and keep individual sub-exposures shorter than instinct suggests — a passing satellite then ruins one short frame instead of one long one. Watch the Moon too: a bright Moon above the horizon washes out deep-sky detail regardless of satellites, which is why the tool reports tonight's Moon alongside the trail forecast.
The standard defence is statistical. Capture many sub-exposures and stack them with outlier rejection — sigma (kappa-sigma) clipping, or a Winsorised/percentile reject in your stacker. A trail appears in only one frame out of dozens, so the rejection algorithm treats it as an outlier at those pixels and discards it while keeping the real signal. More subs make the rejection cleaner. Capturing in the quieter shadow window simply means fewer trails to reject in the first place.
The hours around local midnight, when Earth's shadow has climbed high enough to eclipse most satellites overhead. The exact window depends on your latitude and the season — the tool above computes tonight's cleanest stretch for your location.
Because that is when your sky is already dark but satellites high above are still sunlit. With your ground in shadow and the spacecraft in daylight, they reflect sunlight straight into your camera and record as streaks.
It means the fewest sunlit satellites bright enough to trail. The tool counts the full modelled catalogue — including Starlink and the other megaconstellations — and grades how busy each part of the night is, flagging when it finds a genuinely clear stretch.
Stack many sub-exposures with outlier rejection — sigma/kappa-sigma clipping or a percentile reject. A trail shows in only one frame, so it is rejected as an outlier while the real signal survives. Shorter subs and more of them make this cleaner.
They are the largest single source by sheer number, and the count is still climbing toward tens of thousands. But Kuiper, OneWeb, Qianfan and Guowang all add to the total, and the very bright AST BlueBirds are a disproportionate problem for their size. The tool models all of them.
For deep-sky imaging, often more. A bright Moon above the horizon raises the whole sky background and buries faint detail. That is why the tool reports the Moon's illumination and whether it is up during your dark window, alongside the trail forecast.