Thats a good object for a start!
Get yourself a copy of Deepskystacker..(freeware)
You can then shoot in RAW with your Nikon.
Besides what Dennis has mentioned above, you can make yourself up some diffraction spikes with some fishing line or thin wire. Just take test focus shots and check the spikes and refocus until they become one very thin line. Then you have good focus. Like the hartman mask if you are getting away from focus you will have a double spike and the closer you get they merge into each other till it is razor sharp!
cheers Gary