Also, even if they are well-exposed, the exposure computer may introduce color shifts and improper exposures from what you might expect. A good lab is a real treasure. They develop and mount slides in 3 hours and follow directions well and enough professionals and astrophotographers use them that they know what they are doing. The sturdier the better. Make sure that you can point them upwards without the handle hitting other parts of the tripod.
Taller ones make it easier to look through when composing.
Like telescope mountings, a stiff sturdy mount makes use of the camera that much easier and more efficient. The ones that you can lock with one hand is great as it frees up your other hand for starting the timer. If you want to get a little more complicated, divide the result by the cosine of the declination, since the apparent rotation of stars nearer the pole is slower. You can always expose less for bright objects like the Orion Nebula, but follow this relation for the maximum exposure. A crescent moon needs about a stop or two more exposure than full moon.