This is unlocked for completing Chapter 8, the final chapter of the game.
You begin at your car, search it to get a phone number on a piece of paper you'll need later. Walk towards the stairs up ahead where you'll see a dead guard and a crowbar, which you'll need to pick up.
Climb the stairs and enter the observatory, using your crowbar to break the door open.
You'll need to climb more stairs inside to get to the top level. In this room you'll need to pick up a coin on the table as well as active the power supply in the center of the room to turn the lights off.
Next you'll need to exit the observatory and go down the stairs to the phone booth you passed on the way up. Place the coin into the phone and dial the number you picked up earlier.
After making the phone call, you'll need to return to your car and open the trunk. Upon opening the trunk, inspect the contents to end the game and unlock the achievement.