Photography Calculators

Astrophotography

Star Trail Stacking Calculator

Star trails are many short frames, not one hour-long exposure that cooks the sensor. Frames = floor of total seconds over interval. The sky moves 15° per hour at the equator, times cos(declination). The 30 fps clip length is frames/30 if you also want a tiny video. Leave a second or two of gap so the buffer keeps up — then inspect a test shot for gaps in the trail.

Inputs

Units
min

Wall-clock minutes you will leave the camera running.

s

Time from start of one frame to the next. Must be longer than shutter plus write time.

°

0° at the celestial equator (longest trails). ±90° at the poles (almost no arc).

Results

Frames
120

Whole intervals that fit in the shoot, rounded down.

Trail arc
15.00°

Trail length on the sky: 15°/h × hours × cos(dec). Around the pole each star still sweeps 15°/h of its circle.

Clip at 30 fps
4.00s

Frames ÷ 30, if you also render the sequence as a clip.

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Calculation method

Frame count is the integer number of intervals that fit in the total minutes. Sky rotation is 15 degrees per hour at declination 0°, falling with the cosine of declination so polar stars barely move. A 60-minute shoot at the equator is a 15° arc. Around the pole every star still sweeps 15° of its circle per hour — an hour is 1/24 of a full circle — but the trail’s length on the sky shrinks with cos(declination). If you play those frames at 30 fps, clip length is frames divided by 30 — a 120-frame hour is a 4-second clip, not a movie.

Exact formula

frames = floor(totalMin×60 / intervalS). trailArcDeg = 15 × (totalMin/60) × cos(dec). videoSecondsAt30fps = frames/30. Limits: 15°/h rounds the sidereal 15.04°/h; polar alignment, field rotation, gap-free stacking and shutter-open time inside the interval omitted.

One long exposure vs a stack

A single 60-minute frame is hot pixels, skyfog and a ruined take if someone walks through. Short frames stack, let you drop planes, and keep the sensor cooler.

Foreground

Light the foreground once at the start or end and blend. Do not raise ISO on every trail frame or the sky will cook.

Frequently asked questions

Shutter versus interval?

Interval must be longer than shutter. If you shoot 25 s, use 27–30 s interval so the camera can write. A 25 s shutter on a 25 s interval will skip frames.

Gaps in the trail?

Usually interval too tight, a long RAW write, or long-exposure NR (turn it off). Stacking software can blend small gaps; it cannot invent an hour you did not shoot.

Aim at Polaris?

Declination near +90° (or −90°) makes circles around the pole and a tiny arc value. Every star still sweeps 15° of its circle per hour, so an hour draws 1/24 of a circle and six hours a quarter; the arc value is the length on the sky, which shrinks with cos(dec). Aim equator-ward for long streaks across the frame.

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