Checklist to follow when photographing stars:
1) Set ISO to about 800.
2) Set camera to Manual Focus, view the stars using the view finder and make sure they are sharp. The focus shouldn't be touched once you do this.
3) Set shutter speed using the thumb rule 600/focal-length to get the number of seconds the shutter can be left open without much visible streaking caused by rotation of the earth.
4) Make sure that the aperture is set to as wide as possible.
5) When using a timer or a remote trigger, change to bulb mode. If not, make sure you have the delayed shot setting set. This way you will avoid passing any vibrations from the hand to the camera.
Other considerations.
1) I find focal length of 100 mm to be too large. A wider lens is better when photographing stars.
2) I use a cheap timer to capture a number of pictures in sequence. Pictures so obtained can be stacked using software such as star trax for showing star trails.
3) Use Google sky night on Androids to track constellations.
4) You can get a good picture of the Orion Nebula by pointing to the Orion constellation.
Ps: I intend on modifying this as and when I make changes to my process.
