What is the “100 Rule” in astrophotography, and how do I use it?

The “100 Rule” in astrophotography is a guideline used to determine the maximum exposure time before stars begin to trail due to Earth’s rotation. It states that the exposure time should not exceed 500 divided by the focal length of the lens or telescope (in full-frame equivalent). For example, if using a 50mm lens on a full-frame camera, the maximum exposure time would be approximately 10 seconds (500 / 50 = 10). Adhering to this rule helps prevent star trailing and maintain pinpoint star images.