The backdrop that halfloaf created looks really great...but in your scene, it looks flat and lacks depth. I almost looks like a wall with a picture of space on it rather than a door looking out into space.

The sad part is I have absolutely no idea how to improve that. Maybe try taking the blur out of the backdrop. If it's a plane with the image loaded into the color field, then there is a filter selection near the filename textbox. If you turn the filter off, it should make the image crisp. Not sure that will do the trick though. I'll google some space images and see if I can come up with anything else that might help.