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What a star tracker does
A star tracker is a small motorised mount that turns your camera at the same rate as the Earth turns, in the opposite direction. While the Earth carries an ordinary tripod away from the stars, the tracker carries the camera with them, so stars stay points even during exposures of several minutes. That makes deep night photography possible without a telescope mount. The catch is that no affordable tracker is perfect, and this calculator tells you how far you can push yours.
Trackers fail in one specific way: drift. Gears have small errors, polar alignment is never exact, and wind nudges the camera. The result is that stars slowly creep across the frame even while tracking, a few arcseconds per minute. Whether that creep ruins a photo depends on your pixel scale, which is how much sky each pixel covers. A long lens with small pixels has a fine pixel scale, so a few arcseconds of drift crosses several pixels and blurs the star. A wide lens with large pixels forgives the same drift easily. The calculator combines your focal length, pixel size, and drift rate into one safe exposure time.
How the calculation works
First it finds your pixel scale: 206.265 times your pixel size in micrometres, divided by your focal length in millimetres. That gives arcseconds per pixel. A 100 mm lens with 4 micrometre pixels gives about 8.3 arcseconds per pixel. Then it allows two pixels of drift, which is a sensible blur limit: a star smeared across two pixels still looks sharp at normal viewing sizes, but more than that starts to look soft. Finally it divides that allowed drift by your tracker’s drift rate. At three arcseconds per minute, 16.5 arcseconds of allowance lasts about five and a half minutes.
The drift presets cover typical mounts: about one arcsecond per minute for a good, carefully aligned tracker, three for an average one, and six for a basic one. If you have measured your own mount’s drift, use the closest preset and treat the result as a planning figure, not a promise. Polar alignment dominates everything: a tracker aligned within a degree of the pole drifts far less than one aligned sloppily, so spend the extra minutes on alignment before blaming the mount.
A worked example
Take a 100 mm lens, 4 micrometre pixels, and an average tracker drifting at three arcseconds per minute. The pixel scale is 8.25 arcseconds per pixel, the two-pixel allowance is 16.5 arcseconds, and the safe exposure is 5.5 minutes. That is long enough for serious deep-sky work with a short telephoto. Now change only the lens to 200 mm. The pixel scale halves to 4.1 arcseconds per pixel, the allowance halves to 8.25 arcseconds, and the safe exposure halves to about 2.75 minutes. Double the focal length, halve the safe time: that is the iron rule of tracked photography.
Change the camera instead. A camera with 6 micrometre pixels on the same 100 mm lens has a pixel scale of 12.4 arcseconds per pixel, so the same tracker allows over eight minutes. Bigger pixels forgive more drift, which is one reason older, lower-resolution cameras sometimes seem easier for tracked work. And with a good tracker at one arcsecond per minute, the original setup allows over sixteen minutes, which is longer than most photographers need before sky glow, not drift, becomes the limit.
Getting the most from your tracker
Balance the mount carefully. An unbalanced camera strains the motor and increases periodic error, the rhythmic gear wobble that is the main source of drift. Align on the pole star as precisely as you can, using the mount’s polar scope or a phone app, and recheck alignment if you bump the tripod. Shield the setup from wind, because a gust moves the camera far more than gear error does, and hang a weight from the tripod’s centre column on soft ground.
Test before committing. Shoot one frame at the calculated safe exposure and zoom in on the stars at full size. If they are points, try a longer exposure; if they are dashes, shorten it. Stacking many tracked frames is the standard workflow: ten frames of three minutes give you thirty minutes of total light with far less risk than one thirty minute frame, and stacking software can align small drifts between frames. Keep individual frames inside the safe limit and let stacking do the heavy lifting.
Frequently asked questions
Why are my tracked stars still dashes? Usually polar alignment, not the tracker. Realign carefully, check balance, and confirm the mount is set to the correct hemisphere and tracking rate before suspecting a fault.
Does the tracker fix field rotation? Only partly. A single-axis tracker corrects rotation near the pole it points at, but wide fields far from the pole still rotate slowly. Keep very wide tracked exposures modest, or use a two-axis mount.
Can I use a tracker for star trails? You can turn tracking off and use it as a solid tripod, but tracking itself stops trails. Trails and tracking are opposite techniques, so choose one per shot.
Use the calculator above with your real lens, pixel size, and mount quality, test one frame at the result, and remember the framing: tracker figures are starting points, and alignment, balance, and wind change the true limit on any given night.
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