interval shooting

what would be a reasonable time to set between shots for star trail at f5.6, which seems to be the recommended aperture.

Hi, depends on the focal length (divide 600 by it and you get when trails roughly start ie 60 seconds at 10mm and two seconds at 300mm). I've seen full circle ones that are three hours. I personally try to take wide skies without trails. Loads on the web about this subject, mostly from people living near deserts away from town light pollution