How To Calculate Star Trails
This star trails tool will give an estimate to how long an exposure you need for star trails to form at a given focal length and target declination.
Without tracking or a motor driven mount, long photographic exposures will create star trails as the Earth rotates. This gives the appearance of stars streaking across the sky, which can make interesting photos. Sometimes however, you may want to take a photo of the stars without trails. How long an exposure can I use before stars start to trail? or in reverse How long do an exposure do I need before star trails are shown?.
This simple calculator will tell you how many seconds before stars start to trail. Any exposure above this will have star trails, below should have no trails.
This calculator will only give a rough estimate based on focal length of the camera and the targets declination in degrees. Actual results will vary between lenses and cameras, so a little experimentation will be required. This tool will give a ball park figure.