How to Use the Star Trail Session Planner

Enter the declination of the sky you will frame, and the planner gives the trail length, the frames to shoot and how much of the frame the trails cross.

Open the Star Trail Session Planner

What this tool answers

How long will my star trails be for the part of the sky I frame and the time I shoot?
How many frames do I shoot, and how much total exposure do they add up to?
How much of my frame width will each trail cross?

For why trails look different near the equator, read Star Trails in the Tropics.

What you see on the screen

Observer: place name, coordinates and a Use my current location button. The page starts from your saved location, your approximate network location, or Georgetown. There is no place list or latitude box.
Session setup: a Demo preset button, then Camera (six presets, Sony A6600 by default), Focal length (mm), Target declination (°), Session length (min), Shutter / frame (s), Gap (s) and an Update plan button.
Results: an arc sketch (not to scale), Trail arc, Frames to stack, Effective exposure, FOV width, a Trail vs frame width bar and a summary line.
Low-latitude tip: which stars circle the pole from your latitude.

The defaults match the demo: 24 mm, declination -30°, 60 minutes, 25 s frames and a 1 s gap.

Step by step

At your site, tap Use my current location, or tap Demo preset to load Georgetown (this also resets every field).
Choose Camera and type your Focal length (mm).
Enter the Target declination (°) for the part of the sky you will frame. The table below helps.
Enter Session length (min), Shutter / frame (s) and Gap (s).
Tap Update plan. A field also applies when you leave it; values outside the allowed range are ignored.
Read Trail arc and the Trail vs frame width bar.
Set your intervalometer to Frames to stack shots at Shutter / frame (s), with Gap (s) between them.
Stack the frames in Sequator or StarStax, as the results line suggests.

The address bar keeps your settings, so you can bookmark or share a plan.

Picking a target declination from Georgetown

Declination is a star's distance north or south of the celestial equator, and it sets the shape of its trail. See Finding Objects in the Night Sky Without Polaris for a refresher.

| Where you point | Target declination | What the trails look like | | --- | --- | --- | | Due east or west, low | 0° | Straight, nearly vertical streaks, the longest trails | | Straight up | about +7° | Long, nearly straight lines | | Due south, high (about 54° up) | about -29° | Long, gentle curves | | Due south, low (20° to 30° up) | -53° to -63° | Shorter arcs bending around a pole below the horizon | | Due north, low | +83° to +89° | Tiny circles around Polaris, touching the horizon |

Rule of thumb: from Georgetown a star crosses due south at 83.2° plus its declination above the horizon, so Acrux (-63°) reaches about 20°.

Real star trails from Georgetown or Lethem tonight: aim the camera and read the declination at the middle of the frame.

You will frame due south, about 54° up, from Georgetown. Which Target declination do you type?

+54°
-54°
About -29° (the right answer)
0°

The planner wants declination, not altitude: due south a star stands 83.2° plus its declination above the Georgetown horizon, so 54° up is about -29°.

How to read the results

Trail arc: how long each trail is on the sky, in degrees. The planner uses 15 arcseconds per second times the cosine of the declination, so the trail grows 15° per hour at declination 0° and about 6.8° per hour at -63°. The demo gives 13.0°.
Frames to stack: the session length divided by one shutter plus one gap, rounded down. The demo gives 138.
Effective exposure: frames times shutter, in minutes. The demo gives 57.5 min.
FOV width: the lens's horizontal angle on that camera, and the trail as a percentage of it. The demo gives 52° and about 25%.
Low-latitude tip: the declination a star needs to be circumpolar at your latitude, whether your target qualifies, and the advice that framing south usually gives the most satisfying arcs.

The planner's own numbers, starting from its demo: change the declination, the session and the frames.

The real Star Trail Session Planner, step by step: load the demo preset, read the trail arc and the frames to stack, then try a two-hour session.

The planner always plans a stack of short frames, never one long exposure.

Using it from Guyana

Polaris sits 6° to 7° above the north horizon at Georgetown and 3° to 4° at Lethem.
Only stars above declination +83.2° are circumpolar at Georgetown (+86.6° at Lethem), so northern circles are tiny and touch the horizon.
The south celestial pole is 6.8° below the Georgetown horizon. Southern trails curve around a pole you cannot see.
Stars south of declination -83.2° never rise at Georgetown, and the planner does not warn you.
Humid nights fog lenses on long sessions. Check the Dew Risk Calculator and pack a lens heater.

What it does not do

It has no date or time, so it cannot tell you whether your target is up, where the Moon is, or when darkness starts. Use Sun/Moon Times & Dark Window.
It has no weather data. Check Stargazing Weather.
Location only changes the Low-latitude tip. The trail numbers are the same anywhere.
It gives no ISO or aperture advice.

Common mistakes

Pointing north for big Polaris circles. From Guyana they are small and low.
Typing the altitude of your framing instead of its declination.
Setting a long Gap (s), which leaves breaks in each trail.
Planning a session longer than the dark window or your battery.

What does a long Gap (s) between frames do to the stacked trails?

Makes them longer
Makes them brighter
Leaves breaks in each trail (the right answer)
Nothing: the stack fills the gaps

The stars keep moving during the gap, so a long one leaves a dashed line instead of a smooth trail.

Lessons: Star Trails in the Tropics, Finding Objects in the Night Sky Without Polaris
Tools: Milky Way Season Planner, for pairing trails with the core

How to Use the Star Trail Session Planner