What environmental costs for mercury?
It isn't like the mercury used to float instruments / rooms / etc. leaks out into the environment, or is difficult to capture down the road when the facility is demo'ed.
Besides, if you want to worry about mercury getting into the environment you first need to shut down all coal burning power plants operating in the region, and clean up every fly ash dump. Before those are taken care of, worrying about "environment costs" from using mercury to float a telescope is ludicrous!