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.

Setup is the hidden cost. Getting a mount levelled, polar aligned, cooled and focused takes most people thirty to sixty minutes, and packing down in the dark takes another twenty. A ninety-minute gap can easily be entirely consumed by logistics.

Visual observing has a far better ratio. A telescope that lives assembled in the garage can be on a target in ten minutes, which makes short gaps genuinely worth chasing — and makes a marginal forecast much more attractive from home than from a site an hour away.

Gap reliability matters as much as gap length. A break forecast between two blocks of heavy cloud is a coin toss on exact timing; the same two hours forecast as the tail of a clearing trend is far more dependable. The hour matrix shows the shape either side of the window, which is the information you need to judge that.

The window finder builds this in: each mode reports the longest unbroken run that clears the usable threshold under its own weighting, plus what opens and closes it.

REVIEWED 2026-08-12 · SOURCES: MET Norway, sun and moon positions computed on our own servers