Do I still need the 500 rule if I'm using a star tracker?

No — a star tracker physically rotates the camera to counteract the sky's apparent motion, which removes the trailing constraint that the 500 rule and NPF rule both exist to predict. With tracking engaged and functioning correctly, exposures of several minutes become genuinely possible without the star-point streaking either rule was built to warn you about.

Why the Constraint Disappears, Not Just Loosens

Both rules model how quickly a stationary camera falls behind the sky's apparent rotation — a star tracker eliminates that gap directly by matching the camera's motion to the sky's, rather than simply tolerating more of the mismatch the way a wider lens or lower-resolution sensor does. This is a qualitatively different solution, not a more generous version of the same math, which is why star-tracker exposure times are calculated from tracking accuracy and total light needed, not from focal length divided by any constant.

What Actually Limits Exposure Time With a Tracker Instead

Once trailing is off the table, the real limits become tracking accuracy (small mechanical or alignment errors accumulate over a long exposure), foreground blur (the ground itself is no longer stationary relative to a tracked, sky-following camera, so a foreground element in frame will streak instead), and simple practical patience — most trackers can hold reasonably tight tracking for several minutes per frame before drift becomes visible even at pixel level.

For any shot without a tracker, the trailing limit is still real — check it in the Astro 500/NPF Rule Calculator.