Kalil,

If you want "no atmosphere," go to load atmosphere: effects: others: to find "no atmosphere."

1. Otherwise, just render
2. click the save image icon  OR go to FILE: Export picture: Save color picture
3. pick PNG, TIFF, TGA or any other format that supports alpha channels
4. MAKE SURE you click "embed alpha info."

That's it.

I hope that's what you were asking.