*Possibly* Crack-Pot Idea
Have millions of users take random photos of the night sky with their digital cameras. Let them send them in to a central server. The server uses software to automatically align/rotate them with each other and group by time of year. It averages them out in order to get a very precise image of the entire night sky. It would probably want to use this precision to be able to remove the effect of light pollution from the result.