Enter the Hall of Wonders (NW Baldur's Gate) with your thief and take the guided tour. Once that's over, position yourself to the northeast of the container with the telescope in it and wait to be in the clear. Quickly pick the lock, grab the telescope, and book it northeast into the High Hall of Wonders and then outside.
You'll get a notification that the guards will be summoned but it doesn't actually happen and the game then treats it as if you stealthily stole it and no one noticed.
The item can then be brought to Brevlik in the Elfsong Tavern in East Baldur's Gate for a quest completion.
Alternatively you can wait and go into the Hall of Wonders after dark. All of the guards and other NPCs (except Alora who is a potential companion) will be gone in this part of the museum. Simply walk up to center container with the telescope, lockpick, and take the telescope.