# 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?

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.

• It will vary depending on what part of the sky your camera is pointed, just as the 1/500 rule of thumb varies. Stars near the celestial equator will move in the frame at a faster rate than stars near the celestial poles. Sep 15, 2020 at 23:50
• Not an answer to this question, but perhaps an answer to the underlying problem that caused you to ask this question: A DIY barn door tracker Sep 15, 2020 at 23:59

## 1 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.

• Oh that calculator looks super useful, thanks for the response :-)
– Benj
Sep 15, 2020 at 12:42
• The google ads covering parts of your content make those portions unreadable. Sep 15, 2020 at 23:53
• I don't see any problem, Chrome and Firefox are fine with it at any screen width. It is probably not good content here, is there a way we can discuss it? Sep 16, 2020 at 1:06