I have a Canon T1i with an EF18-55mm lens that came with it.How do I set exposure time for 20 or 30 seconds.Ihave it mounted on top of my telescope and I am trying to take pictures of moon and planets.Can somebody please explain what f/stop, iso is. For instance I have camera on now on M with top line of screen showing 1/20 F5.6 iso200 what does all this mean between my telescope forum and intruction book that came with camera I am now totally confused.