Equivalent of the 500 rule for working out how many frames you can take with a given focal length before resetting/moving a manual mount?

Question

I'm new to astrophotography and I don't have a tracker.  I just have a light pollution filter, an intervalometer and a camera on a tripod with a few different prime lenses.  I'm using deep sky stacker to make photos of deep sky objects.
I've read about the 500 rule which has been useful for estimating the maximum length of my exposures with my different lenses to avoid star trails.  However, I'd like to use my intervalometer to get a consistent number of exposures before resetting my camera on the subject so that I get a consistent crop factor of wasted image, for stars which have slid on/off the view during my shoot.
Is there a similar rule for doing this?  It seems like it should be possible but I can't figure out the math.

WayneF · Answer

The rotation movement is greater than might be imagined, esp with lenses longer than extreme wide angle.  I would offer the calculator on my site at https://www.scantips.com/lights/stars.html that is an enhancement beyond the 500 Rule.  For a given sensor size and lens, it computes the specified time duration's length of the rotational star trail in both mm and pixels (and degrees and multiples of the CoC limit). There is no way to provide those results here, so I offer the link.
How much is too much for your purpose is left for you to decide, but a tracker will be very useful.

Equivalent of the 500 rule for working out how many frames you can take with a given focal length before resetting/moving a manual mount?

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