Star trail calculator

Give it the session you are planning — how long, what frame length, which lens, where you are pointing — and it returns the trail you will actually get: its length on the frame, a sketch of its shape, the number of frames, and whether the gap between them will break the trail into dashes. Free, no account, nothing stored.

What it does, and why it is here

A star trail session costs hours of standing in the cold, and everything that can go wrong is invisible while it is going wrong. A single 30-second frame looks identical whether the night will stack into a photograph or into a smudge. The two failures only appear at home, once the frames are stacked and the night is over.

The first is trails too short to read as trails. That is decided by how long you stay and how wide you shoot — and by where you point, which is the part people forget: a star on the celestial equator draws the full arc, while one near the pole barely moves.

The second is the gap nobody thinks about: the dead time between two frames while the camera writes the file to the card. The shutter is closed and the sky keeps turning, so every gap is a missing segment. At a short focal length it is a fraction of a pixel and vanishes; at a long one it is several pixels, and the finished trail is a dotted line.

So this page answers both before you go, from the gear you actually own, and adds the housekeeping the night needs: how many frames, how much card, how many batteries, and what time you will be packing up.

What you fill in

• Session length, in minutes — how long you intend to stand there. It sets the arc, and the arc sets the trail.
• Frame length, in seconds — the individual exposure, typically 15 to 30 s. Short enough and each frame is also usable on its own.
• Interval between frames, in seconds — the number of this tool. Not the intervalometer setting people usually think of, but the dead time while the file is written. This is what decides line or dashes.
• Focal length, in mm, and sensor format — together they give the field of view, which is what turns an angle in the sky into a length on your frame.
• Resolution, in megapixels — so the answer can be given in pixels and as a share of the frame, not only in degrees.
• Declination, in degrees — where you are pointing. 0 is the celestial equator, where trails are longest and straightest; 90 is the pole, where they close into circles.
• Start time, optional — to be told when the last frame ends rather than working it out in the dark.
• Size of one frame in MB and frames per battery, both optional — the two numbers that turn the plan into a packing list.

What you get back

• The sky turns — the arc covered over your session, in degrees. One degree every four minutes at the equator, less the closer you point to the pole.
• Trail length, in pixels and as a share of the frame. The share is the number that means something: “524 pixels” says nothing until you know the frame is 6000 wide, while “8.7% of the frame” tells you immediately that half an hour at 24 mm will be a smudge.
• A sketch of the trails — drawn for your actual field of view and declination, and it changes shape: closed circles when the celestial pole falls inside the frame, arcs of large circles when it does not. This is the composition question answered before you leave the house.
• Gap between frames, in pixels, with a verdict in plain words: small enough to disappear once stacked, or large enough to show as dotted lines.
• Longest continuous interval — the interval to stay under for an unbroken trail, at this focal length and this declination. The single setting to take into the field.
• Frames to shoot and light actually collected — the second is not the first: the interval steals real exposure time, which is why a long gap costs twice.
• The session plan — when it ends (marked next day when it crosses midnight), the card space needed, and the number of batteries, counted on the frames actually shot rather than on the round minute.

What it does not do

It knows the geometry, not the sky above you. It does not know your light pollution, your Moon, your cloud or your horizon — a session it says will work can still be ruined by a bright Moon, and choosing the dark nights is a separate question.

It does not stack the frames either: it plans the shoot, and the stacking happens afterwards in software of your choosing. And it assumes a rectilinear lens with no correction applied; a fisheye bends the trails in a way no single number describes.

How long a session, and at what focal length

The two settings that decide whether there is a photograph at the end, for a 24 Mpx full frame.

The last row is the trade every star trail photographer makes. Point at the pole and you get the concentric circles everyone recognises — but the stars near the centre barely move, so the same two hours draw a tenth of the trail. Point at the equator and the trails are long and straight after an hour, with no pivot to compose around.

Questions

Is this calculator free?

Yes. No account, no e-mail, nothing stored — the session you describe is used to answer and then forgotten.

How long a session do I need?

Long enough that the trail is a recognisable share of the frame. At 24 mm on the celestial equator, half an hour reads as a smudge and two hours reads as a photograph; nearer the pole, or wider, multiply. Ask the tool with your own lens rather than trusting a rule.

Should I shoot one long exposure instead?

Rarely. A two-hour single frame collects two hours of sensor heat, two hours of light pollution, and two hours of risk that one car’s headlights ruin everything. A few hundred short frames stacked afterwards give the same trail, recover from a single bad frame, and let you stop when you have enough.

What does the gap figure mean, and how do I get it down?

It is how far the sky moves, in pixels, while the shutter is closed between frames. Two hours at 24 mm with a one-second gap is 0.3 px and invisible; the same session with an eight-second gap is 2.3 px and visibly dashed — and it also loses 43 frames of collected light. To shrink it: turn off long-exposure noise reduction, which is the usual culprit (it shoots a second dark frame as long as the exposure, so a 30-second frame becomes a 60-second cycle); shoot to a fast card; set the intervalometer to fire as soon as the buffer allows; turn off image review and in-camera lens corrections.

How do I stack them?

Any stacker that offers a lighten or maximum blend: each pixel keeps the brightest value across the sequence, which is exactly what draws a trail. Shoot a handful of dark frames at the same length and temperature for the noise.

Which direction should I point?

North (or south, below the equator) for circles around the pole, and expect to stay longer. East or west for long straight trails in less time. The sketch in the answer shows which of the two you are about to get.

What about aircraft and satellites?

They arrive as straight lines through the frame. Because you shot hundreds of separate frames, the fix is to drop the two or three frames that carry them before stacking — something a single long exposure would not have allowed.

Does it account for lens distortion?

No. It computes the geometric field of view for a rectilinear lens; a strong wide-angle or a fisheye stretches the corners, so trail lengths near the edge of the frame are approximate.

Sources and further reading

• Sidereal day — why the sky turns 15° an hour and not 15° of clock time.
• Declination — the coordinate the field asks for.
• Star trail — the technique, and the shapes it produces.