AstroGearFit
What sets the limit
Star trailing depends on pixel scale, tracking error, declination, polar alignment, guiding, and the amount of elongation you are willing to accept. The same mount can support different exposure lengths at different focal lengths.
Use the trailing calculator for a first estimate, then test a short sequence and inspect stars at full resolution.
Longer is not automatically better
Long exposures can improve efficiency when read noise and sky brightness make them worthwhile. But a long exposure that is ruined by wind, guiding loss, or a passing satellite is not more useful than several shorter clean frames.
Balance histogram position, star shape, saturation, and the camera’s dynamic range.
Treat the result as a starting point
The tool does not know your tripod, cables, seeing, focus, or mechanical flexure. Use it to narrow the test range, not to promise that every frame will be sharp.
Important: These calculators are planning aids, not guarantees of equipment performance or mount safety. Check manufacturer specifications and test your own system.