[TurboFan]

Look for something no slower than f/4. You really want f/2.8 in a zoom, or f/1.4-f/1.8 in a prime lens. Are you looking for a wide angle or telephoto, prime or zoom? That's going to affect your repsonses. There's not a whole lot of point in a tele at night, but you can do a few cool things with them to control perspective. Remember too that any lens is going to have a 1.5-1.6 multiplier on the focal length with your camera. An 18mm won't be very wide at all.

You'll also want to get familiar with mirror lockup if your camera supports it. Mirror slap can ruin a long exposure. I don't know much about the Canon, but you may also want to look into something to block the eyepiece as well. Light can enter through the eyepiece at night, and give a haze to the whole photo. Not usually an issue in a modern camera, but if you're shooting exposures over 1 second, or you are around an area with headlights, they can do some harm through the eyepiece.