Thanks for the advice on using the dark frame technique. I see that my star trails stacking program allows me to insert a dark frame. Please excuse my lack of knowledge on the subject, but how do you properly shoot a dark frame. Do you simply take a picture with the lens cap on, and if so, do you use the same exposure setting and exposure time as your other star trail shots. i.e. does does the dark frame have to be a 30 second exposure if the other shots are 30 second exposures? Thanks.