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Guides

Organised by the decision you are making, not by topic. Every one of these exists because it answers a question the forecast raises but cannot answer on its own.

Read the forecast

Transparency vs seeing Transparency is how much light gets through the atmosphere. Seeing is how steady it is. They are independent, they often move in opposite directions, and which one matters depends entirely on what you are pointing at. Low, mid and high cloud, and why we split them A single "60% cloud" figure hides the thing you need to know. Sixty per cent low cloud is a wall. Sixty per cent high cirrus is a veil you might still image through, and it will quietly ruin your data while looking fine to your eye. Civil, nautical and astronomical twilight Astronomical darkness begins when the sun is 18° below the horizon. Above that, there is still measurable skyglow, and faint deep-sky work is compromised even though the sky looks black. What the GoodSeeing score actually measures One number from 0 to 100 per hour, combining cloud, transparency, seeing, moonlight and darkness. Cloud gates everything else — no amount of steady, dark air rescues an overcast sky. Why the forecast changed since you last checked Weather models rerun several times a day on new observations. A night three days out routinely moves by twenty points between runs. A night tonight rarely moves much at all.

Plan a session

How moonlight affects deep-sky observing Moonlight raises the sky background across the whole sky, and faint extended objects are the first casualties. What matters is not just the phase but whether the moon is above your horizon at all. Planning around the lunar cycle Pick the dates from geometry, then check the weather when it gets close. The dark window moves about fifty minutes later each night, and the usable moonless stretch collapses to nothing around full moon. When a clear gap is worth driving for Ask how long the gap is relative to your setup time and your exposure length. A two-hour gap is a real session for a visual observer and close to worthless for someone doing four-minute subs on a target that needs three hours. Red light and dark adaptation Full dark adaptation takes twenty to forty minutes and one glance at a white phone screen resets most of it. Use deep red light, as dim as you can work with.

Match conditions to the target

Conditions for planets and the Moon Seeing is everything. Darkness and moonlight barely matter, and a hazy, humid night that ruins deep-sky work is often the steadiest night of the year for planetary detail. Conditions for galaxies and faint nebulae Everything that raises the sky background is your enemy: moonlight, high cloud, haze and twilight. Seeing matters far less than observers expect. Why thin high cloud ruins long exposures Cirrus you can barely see will halo your bright stars, lift your background and vary between frames. Visual observers shrug it off. Long exposures integrate it, so it accumulates.

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