I was actually quite lucky with color negative, I used Fuji Superia 800 and my exposures were up to 1 or 1.5 minutes. The high sensitivity of the film helped reveal the faint lights in less-than-ideal setting with way too much light pollution and my aperture was f4 or f5.6 with a super-wide angle. I' m sure if you manage to find a really dark place, ISO 200 or 400 will work nicely.