'Star trail' shots actually are FINE with smaller apertures... if it's just the stars you need... keep it around f/11-f/16 and you'll be fine. It's wide field and other astro work you need to be wide open for (5.6 won't cut it - and you need to use hypersensitized film which is a custom setup these days). Flatness of field is ultra critical for that work - but for star trails - don't worry - your min exposure to see any significant trails are going to be at LEAST one hour (will cover 1/12th of the distance across the night sky)...