Goto definately makes it easier. If however you need to push to without an AN then it's a case of getting as close as you can with the finderscope, take you're focusing shots (either DSLR Focus (your mad if you don't, unless you've got a 20D) or 10 sec snap, check focus, refocus, 10 sec snap, check focus, refocus, 10 sec snap, zoom in, check focus, refocus, 10 sec snap, zoom in, check focus, ....yadda yadda until you're focused), then take a shot at max ISO for 0.25 to 0.5 of your calculated imaging time to see if you are centered on the object you want. If not then adjust scope and reshoot, adjust again as needed........ finally centered,
RESET YOUR ISO (man the number of time's I've forgotten to do that

) and take your shots.
If you don't have them, get DSLR Focus and a Goto
(Did I make my point

)