Star trail stacking on a Mac, honestly compared
The most capable free star-trail software does not run on macOS, and the search results rarely say so before you have downloaded something. This page is the list, with what each tool actually does, including where this one is the weaker choice.
What runs where
| Tool | Runs on | Blend | Per-frame sky equalisation | RAW |
|---|---|---|---|---|
| Sequator | Windows only | lighten, plus a star-trails mode | light-pollution reduction, mode-scoped | yes |
| StarTrails.de | Windows only | lighten | no | no |
| StarStaX | macOS, Windows, Linux | lighten, with gap filling | no | no |
| aerotimelapse.com | any browser | eight modes, all plain | no | no |
| Siril | macOS, Windows, Linux | general astronomical stacking | background extraction, manual | yes |
| this site | any browser | lighten, float32 linear | yes, per frame, automatic | CR2, CR3, NEF, ARW, RAF, DNG |
Scroll the table sideways for the remaining columns.
Where each one is the right answer
- You have a Windows machine and want the most capable free tool: Sequator
- It ships an intelligent sky mask, foreground detection so the landscape is stacked separately from the sky, RAW input, 16-bit TIFF, light-pollution reduction and a star-trails mode. None of that is claimed here. If you can run it, run it — the reason this site exists is that a lot of people cannot.
- You want a desktop application on macOS and a familiar workflow: StarStaX
- Native, free, long-established, with gap filling and comet mode. It does a plain lighten blend, which is exactly the blend the measurement on this site is about, and its gap-fill amount is a slider you set by eye.
- You want more blend modes and dark-frame handling in a browser: aerotimelapse.com
- Free, browser-local, with gap fill, comet mode, dark frames and plane rejection. Those are real features and this site does not claim them as new. Its blends are plain — no per-frame background equalisation — and its documented plane rejection is the per-pixel temporal kind, which measures badly for trails for a reason worth understanding: a star trail is itself a single-frame spike at every pixel it crosses.
- You are doing serious astrophotography and will read a manual: Siril
- Free, cross-platform, far more capable than any of the above at registration, calibration and background extraction. It is not built around star trails and it is not a two-minute job.
- You want the faint end of a light-polluted night, on any machine, with nothing installed: this
- It equalises each frame's sky background before the blend, which recovers faint trail a plain lighten loses, and it computes the gap-fill radius from your files instead of asking you for it. How much it recovers depends on how much your sky moved — measured on three cameras at between 1.3× and 4.5×, and the tool tells you where your own night sits. It runs in the tab, on macOS, Linux, Windows or ChromeOS, and it uploads nothing.
Why Sequator will not run on your Mac
It is a native Windows application. There is no macOS build, and there is no announced plan for one. People do run it under a virtual machine or a compatibility layer with mixed results — which is why one of the most searched phrases in this whole category is people trying to get it working on a particular Windows version rather than looking for a Mac alternative at all.
What you give up by using a browser
Two real things, and it is better to know them now.
Some RAW files. Canon CR2 and CR3, Nikon NEF, Sony ARW, Fujifilm RAF and DNG are read directly now. Three are not: Nikon High Efficiency NEFs, whose pixels are behind a licensed codec; linear DNGs from Samsung Expert RAW and Apple ProRAW, which the phone has already demosaiced; and Fujifilm GFX medium format, which is too large for a tab. For those, export 16-bit TIFF or PNG from your raw developer in one batch and nothing measurable is lost — on our own measurements 8-bit uncompressed cost nothing measurable against linear 16-bit, though that particular result is currently being re-measured. JPEG does cost something, and the tool says so when you give it JPEGs.
Fujifilm frames are slow, and it is the compression rather than the colour pattern. A compressed X-Trans RAF takes three to five times as long to decode as a Canon or Nikon frame of the same size — the Bayer-sensor GFX bodies pay exactly the same penalty. Budget a Fuji night by measurement, not by analogy.
Memory. A browser tab gets a bounded slice of your machine. This tool never holds more than one frame at a time and keeps a fixed set of accumulators — about 480 MB for a 24-megapixel colour sequence, whether that is 30 frames or 300. If your files are larger than the tab can take, it says so with the number before it starts, rather than failing at frame 180.
Stack a sequence, or read why a plain lighten stack loses faint trails.