I had a look an your website - your observatory is a dome, right? Does it have any ventilation (when closed
)? If not, I can well understand the need to the air-conditioner.
I guess I can construct using a passive ventilation system and add the a/c later if it's not enough. If I add a/c I think I'd like to add solar panels to offset the power cost, but that would be another challenge too (connecting to a move-able roof).