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Free Star Trail Stacker
— Browser-Based,No Install Required
Upload your frames, choose from eight blending modes, apply plane and satellite rejection, subtract dark frames, adjust tone, and export a finished star trail image or a stacking animation video — all inside your browser. No Photoshop. No software to download. No files sent to any server. Every frame stays on your device, which means your shooting data stays private and processing speed is limited only by your own hardware.
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Stack your star frames entirely in the browser — private, no install, no upload. Five blend modes, plane & satellite rejection, and video export of the stacking animation.
Files are sorted by name automatically
💡 Scroll / pinch to zoom · Drag to pan · Space stack · Esc cancel · ←→ navigate frames · D toggle frame
All processing runs in your browser — no images are uploaded or stored. · aerotimelapse.com · Aero Timelapse Studio, Ulm 🇩🇪
The Fundamentals
What is a Star Trail Stacker?
A star trail stacker is a tool that combines a sequence of night-sky photographs into a single composite image, making the movement of stars across the frame visible as continuous arcs of light. Each individual exposure in the sequence captures stars as sharp points. When dozens or hundreds of frames are merged — overlaid in sequence using a chosen blending algorithm — the cumulative rotation of the Earth becomes visible as elegant curves tracing paths across the sky.
A star trail stacker is different from a standard photo editor because it is designed specifically for multi-frame pixel blending rather than single-image processing. Comparing brightness across hundreds of frames and retaining, averaging, or accumulating the correct value at each pixel position requires dedicated logic that Lightroom and general editors do not have built in.
This tool brings that process into the browser. Select your frames, choose how they should be merged, and the stacking engine runs entirely inside your device via the Canvas 2D API. No upload. No account. No cost.
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The Eight Blending Modes
The choice of blending mode determines the visual character of the finished image more than any other setting. Understanding each mode before a multi-hour shoot — not after — determines whether you come home with the result you intended.
Lighten (Maximum Pixel) The standard mode for star trail photography. At each pixel position, the stacker retains whichever frame contains the brightest value. Stars — brighter than the surrounding sky — accumulate across frames as continuous arcs. Background sky brightness stays constant because dark areas never win the brightness comparison. The correct choice for clean, high-contrast trails against a dark sky.
Gap Fill Lighten with a morphological dilation pass applied after stacking. When your inter-shot interval produces small dark gaps between trail segments, Gap Fill uses a configurable dilation radius to close them. An adjustable threshold slider controls which dark pixels are filled — a live preview overlay paints affected pixels green on the canvas before you commit. Configurable from 1 to 5 passes for progressively more aggressive gap closure. Fixing gaps at capture is always preferable; Gap Fill handles what could not be prevented.
Comet The accumulator decays with each successive frame, controlled by a decay percentage slider (60–98%). Stars from the most recent frames glow brightest; earlier trails fade toward darkness. The result is a comet-like effect where each star has a bright head and a fading tail. The direction of the fade reverses if you reorder the frame sequence in the thumbnail strip. Effective for editorial and creative compositions where conventional solid trails feel static.
Javelin A forward and reverse Comet pass merged with Lighten. Each trail tapers from both ends toward a bright central point, producing a javelin or spindle-shaped trail segment. Visually distinctive from conventional star trails and from standard Comet mode — the taper appears along the length of each arc rather than at one end.
Average Every frame contributes equally to the final pixel value. Correct for noise-reduction workflows in static deep-sky imaging, where multiple frames of the same subject are averaged to suppress random noise while preserving signal. Not recommended for trail photography — averaging reduces contrast and produces faint, low-impact trails because bright stars are pulled toward the dark sky value they’re surrounded by.
Additive Accumulates brightness by summing pixel values across all frames, clamping to 255. Produces intense, saturated trails but clips to white quickly on longer sequences unless per-frame exposure is kept conservative. Useful for very short sequences or faint star fields where Lighten would produce trails too subtle to read.
Multiply Multiplies normalised pixel values across frames. Suppresses sky glow by darkening mid-tones while preserving bright trail segments — a useful blending choice in heavily light-polluted shooting conditions where standard Lighten produces trails that compete with the orange sky background rather than standing above it.
Darken The mathematical inverse of Lighten — retains the darkest pixel at each position. Used for specific creative composites and dark-subject stacking workflows rather than conventional star trail photography.
Rejection and Noise Reduction
Plane and Satellite Rejection Detects single-frame brightness spikes — the signature of an aeroplane, helicopter, or Starlink satellite crossing the frame — and excludes those pixels before they contribute to the stack. Operates at three sensitivity levels: Conservative, Medium, and Aggressive. The rejection percentage for each frame is shown in the progress display during stacking. Works most reliably with 15 or more frames at intervals of 30 seconds or shorter, where the single-frame spike pattern is clearly distinguishable from continuous star motion.
Dark Frame Subtraction Load lens-cap exposures — captured at the same ISO, shutter speed, and sensor temperature as your light frames — in the Dark Frames panel. The stacker averages multiple dark frames and subtracts the result channel-by-channel from every light frame before stacking. This removes hot pixels, fixed-pattern thermal noise, and amp glow that would otherwise appear as persistent bright dots running through every trail in the composite. Dark frames make a material difference at ISO 1600 and above, or in sessions longer than 90 minutes where the sensor warms significantly during the shoot.
Frame Management
The thumbnail strip shows every loaded frame as a scrollable row. Click any thumbnail to toggle it in or out of the stack — excluded frames are dimmed. Drag thumbnails to reorder the sequence if your files loaded out of alphabetical order. Range-based bulk selection lets you include, exclude, or invert a group of frames by number without clicking each one individually.
Each thumbnail receives an automatic quality score on load. Laplacian variance analysis flags blurry frames — caused by dew, accidental movement, or focus drift. An overexposure check flags any frame where more than 12% of pixels are blown to pure white. Both issues are shown as badges on the affected thumbnails before stacking begins, so you can exclude problem frames before committing to a full-resolution stack.
EXIF metadata is read directly from each JPEG file using a pure JavaScript parser — no external library required. Shutter speed, aperture, ISO, focal length, and capture timestamp are extracted and displayed. When timestamps are present across the sequence, the total session duration is calculated from the first and last frame automatically.
Frame Management
The thumbnail strip shows every loaded frame as a scrollable row. Click any thumbnail to toggle it in or out of the stack — excluded frames are dimmed. Drag thumbnails to reorder the sequence if your files loaded out of alphabetical order. Range-based bulk selection lets you include, exclude, or invert a group of frames by number without clicking each one individually.
Each thumbnail receives an automatic quality score on load. Laplacian variance analysis flags blurry frames — caused by dew, accidental movement, or focus drift. An overexposure check flags any frame where more than 12% of pixels are blown to pure white. Both issues are shown as badges on the affected thumbnails before stacking begins, so you can exclude problem frames before committing to a full-resolution stack.
EXIF metadata is read directly from each JPEG file using a pure JavaScript parser — no external library required. Shutter speed, aperture, ISO, focal length, and capture timestamp are extracted and displayed. When timestamps are present across the sequence, the total session duration is calculated from the first and last frame automatically.
Recommended Gears for Star Trails
The single biggest upgrade you can make before a star trail session is getting a wide, fast lens. You want something at 14mm or wider on full-frame — it gives you longer NPF-safe shutter speeds (35–40 seconds), which means more light per frame and denser, richer trail arcs in the final stack. The one I’d point you to is the Samyang 14mm f/2.8. It’s fast enough to keep ISO under 3200, covers the entire Milky Way core in one frame, and for the price there’s genuinely nothing that competes. If you’re shooting under Bortle 5–6 skies, pairing it with a Kase Wolverine Night Filter makes a noticeable difference — it blocks the sodium and mercury wavelengths behind that orange glow optically, so the stacker’s Light Pollution Removal function has much less heavy lifting to do in post. Just keep in mind that the Samyang’s bulbous front element doesn’t accept standard screw-on filters, so you’ll need either a clip-in filter that sits inside the camera body or a dedicated rear holder system for it to work.
One more thing I’d genuinely not skip: a dew heater. A COOWOO USB lens heater strip runs off any power bank and keeps your front element just above ambient temperature. It sounds minor until dew hits at the two-hour mark of a 300-frame session and the entire stack is gone — and at that point no algorithm in the world saves it.
Step-by-Step Guide
How to Use the Star Trail Stacker
Step 1 — Shoot your sequence correctly
The quality of the stacked result is determined almost entirely at capture. Shoot in Manual mode with a fixed aperture, ISO, and shutter speed — any auto exposure that shifts between frames introduces visible brightness jumps in the stack. Capture dark frames at the end of the session — 10 to 20 exposures with the lens cap on at the same ISO and shutter speed — while the sensor is still at operating temperature.
Step 2 — Upload your frames
Drag your JPEG, PNG, or WebP files into the upload zone or click to browse. Files are auto-sorted by filename. The thumbnail strip shows every frame with quality badges for blur or overexposure detected on load. Toggle individual frames out of the stack by clicking their thumbnail, or use bulk range selection to exclude a group at once. Drag thumbnails to reorder if needed. EXIF data is read automatically — if timestamps are present, total session duration is displayed.
Step 3 — Load dark frames (optional but recommended)
If you captured dark frames, open the Dark Frames panel and load them. The stacker averages them and subtracts the result from every light frame before processing begins. This step visibly reduces hot pixels and fixed-pattern noise at ISO 1600 and above.
Step 4 — Choose your blending mode
Select Lighten for conventional trail photography. Gap Fill if your interval was longer than ideal and you want the stacker to close dark gaps between segments. Comet or Javelin for creative fading-trail effects. Average for noise-reduction workflows only — not trail photography. The mode description updates when you change the selection so you know what the output will look like before committing to a full stack.
Step 5 — Enable Plane and Satellite Rejection if needed
If your sequence was shot during a busy air traffic period or from a location with Starlink passes, enable Plane/Satellite Rejection and set sensitivity to Medium as a starting point. The rejection percentage per frame is reported in the progress display during stacking — if it's removing more than 20–30% of pixels per frame, reduce sensitivity.
Step 6 — Set processing resolution
Standard (≤ 2500px) is correct for most workflows and covers full-resolution output from sensors up to approximately 36MP. Full Res processes at native dimensions — the memory warning estimates RAM usage before you start, based on frame count and resolution. Preview (≤ 1500px) is useful for testing mode and tone settings quickly before committing to a full-resolution stack. An ETA estimator benchmarks one frame at low resolution and projects total stacking time for your full sequence.
Step 7 — Stack your frames
Press Stack Frames Now. A progress bar shows the current frame filename, plane rejection percentage, and elapsed time. You can watch the trail image build in the result canvas as each frame is added. Press Esc to cancel at any point. Use Space as a keyboard shortcut to start stacking; ←/→ to step through frames in the thumbnail strip; D to toggle the current frame in or out.
Step 8 — Adjust tone
After stacking, Brightness, Contrast, Saturation, Star Color Boost, and Light Pollution Removal controls appear above the result. All adjustments are non-destructive — the stacker keeps a stored copy of the raw pixel data and re-applies tone settings from scratch each time you move a slider. Star Color Boost applies saturation only to pixels above a luminance threshold of 60 — it intensifies star colours without touching the dark sky. Light Pollution Removal samples the 10th-percentile luminance in a 16×16 grid, builds a smooth bilinear gradient map, and subtracts it at your chosen strength. The live Histogram shows RGB and luminance channels after every tone change so you can monitor for clipping.
Step 9 — Compare and export
Use the Before/After slider to compare the first frame of your sequence against the final stacked result. Zoom and pan the canvas using the scroll wheel and drag to inspect specific areas at full resolution. Download as PNG for lossless full-resolution output. Download as JPEG with an adjustable quality slider for smaller file sizes. For the stacking animation — the video showing the trail building frame by frame — set your frame rate, quality tier, and target duration, then export as MP4 (H.264 via WebCodecs on Chrome, Edge, and Safari) or WebM (VP9 via MediaRecorder as a browser fallback). A live file size estimate shows the expected output size before you commit to encoding.
Frequently Asked Questions
What is a star trail stacker?
A star trail stacker is software that combines multiple night-sky photographs into a single composite image using a pixel blending algorithm. The most common algorithm — Lighten — retains the brightest pixel value at each position across all frames. As stars move across the sky during a shooting session, each frame captures them at a slightly different position. The stacker accumulates these positions into continuous arcs showing the apparent rotation of the sky caused by Earth’s rotation.
How many frames do I need for star trails?
Enough frames to cover the arc length you want in the finished image. Stars near the celestial equator move approximately 0.25 degrees per minute of real time. A 15-degree arc — clearly readable as a trail in a finished print — requires roughly 60 minutes of total session time. At a 30-second interval that is 120 frames; at 60 seconds, 60 frames. Use the Milky Way Exposure Calculator to calculate the session duration for any target arc length before you go to the location.
What is the best blending mode for star trails?
Lighten is correct for the vast majority of star trail photography — it retains only the brightest pixel at each position, which means stars accumulate as trails while the dark sky stays dark. Gap Fill is the right choice when your interval was longer than ideal and you want to close the dark segments between trail pieces. Comet and Javelin produce creative fading-trail variations. Average is not suitable for trail photography — it reduces contrast and produces faint, washed-out results.
Should I shoot star trails as one long exposure or as stacked shorter frames?
Stacked shorter frames produce better quality than a single long exposure in almost every practical situation. A single multi-hour exposure accumulates thermal sensor noise continuously, producing hot pixels and colour noise that cannot be removed in post. A stack of shorter frames produces the same visual arc with far less accumulated noise — and each individual frame remains recoverable if the session is interrupted by cloud, battery failure, or accidental movement. For sessions longer than 40 minutes, stacking is the correct approach.
What does Plane and Satellite Rejection do?
It detects pixels that appear significantly brighter in a single frame than in the frames immediately before and after — the characteristic signature of an aeroplane, helicopter, or Starlink satellite crossing the field of view. Those pixels are excluded from the stack before blending begins. The rejection percentage for each frame is shown during stacking. Three sensitivity levels — Conservative, Medium, Aggressive — control how different a pixel must be to trigger rejection. It works most reliably with 15 or more frames at intervals of 30 seconds or shorter, where the single-frame spike pattern is clearly distinguishable from normal star movement.
What are dark frames and should I use them?
Dark frames are exposures taken with the lens cap on, at the same ISO, shutter speed, and sensor temperature as your light frames. They capture the camera’s own noise signature — hot pixels, thermal noise, fixed-pattern glow — without any sky content. The stacker averages multiple dark frames and subtracts the result from every light frame before stacking, removing noise sources that would otherwise appear as persistent bright dots in every trail. Capture 10–20 dark frames at the end of every session while the sensor is still at operating temperature. The effect is most visible at ISO 1600 and above.
Can I use RAW files with this stacker?
The stacker accepts JPEG, PNG, and WebP files. RAW files need to be converted first — export them from Lightroom or Capture One as full-quality JPEG. The recommended workflow is to develop your base exposure on one representative frame in Lightroom, sync those settings across the full sequence, then export as JPEG for stacking. This preserves your RAW processing decisions while feeding the stacker clean, consistent input.
What is the Comet mode and how is it different from Javelin?
Comet mode applies exponential decay to frames as the stack builds. Each successive frame contributes less brightness to earlier positions in the accumulator, so more recent frames appear brighter. The result is a trail with a bright head at one end and a fading tail at the other — the direction reverses if you reorder the frame sequence. Javelin performs both a forward and a reverse Comet pass and merges them with Lighten, producing a trail that tapers from both ends toward a bright central segment — a spindle or javelin shape rather than a directional fade.
What is the stacking animation video export?
During the stacking process, the tool captures up to 120 snapshots of the accumulating image at regular intervals — one for every N frames stacked. These snapshots are assembled into a video showing the trail building progressively from a single frame to the full composite. On Chrome, Edge, and Safari the video is encoded as MP4 using the WebCodecs API. On other browsers it falls back to WebM via MediaRecorder. Frame rate, quality tier, and target duration are all configurable before export, and a live file size estimate is shown before encoding begins.
Why does my stacked image show gaps between trail segments?
Gaps appear when the interval between frames is long relative to the apparent star motion — typically intervals above 20–30 seconds, depending on focal length and declination. The dark period between shutter close and next shutter open shows as an unlit gap in the trail. Enable Gap Fill mode to close these gaps using morphological dilation. The threshold slider controls which dark pixels are treated as gaps — the live preview overlay shows affected pixels in green before you stack. For future shoots, the simplest fix is a shorter interval: 2–5 seconds more than your shutter speed is sufficient, and a fast card such as a V60-rated SD eliminates buffer delays that add unintended gaps.
What does Light Pollution Removal do?
The Light Pollution Removal function estimates the sky background gradient using a 16×16 sampling grid across the stacked image. It finds the 10th-percentile luminance value in each cell — approximating the sky brightness without being skewed by bright stars — bilinearly upsamples the grid into a smooth gradient map, and subtracts it from the image at your chosen strength. The result is a reduction in the orange-brown gradient that dominates suburban skies. The Strength slider controls the subtraction intensity. Start at 50% and adjust — over-correction pushes shadows negative and produces an artificial dark haze.
Can I stack drone footage frames?
No. Star trail stacking requires a completely stationary camera. Any camera movement between frames — including the micro-instability of a hovering drone — causes stars to shift position non-uniformly across the frame, producing smeared or doubled trails rather than clean arcs. Star trail photography must be shot from a tripod-mounted, ground-based camera. For night aerial work, see the Drone Hyperlapse Calculator.
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