Larceny at the Hall of Wonders is a quest provided by Brevlik in the Elfsong tavern in Baldur's Gate East. A telescope has arrived at the Hall of Wonders museum in Baldur's Gate West, and Brevlik wants you to steal it, ideally without killing any guards. Although he only offers a reward of 500gp, this quest also provides 5500XP upon completion.
The best way to complete this quest is to leave the rest of your party outside the Hall of Wonders, and send just your thief in. Pick the lock on the central exhibit, and when you suspect nobody is looking, take the telescope and leave immediately. Although the guards will always follow you, if you steal the telescope when they are far away, you will have a headstart on getting away from them.
There are also a couple of rare gems in display cases in the Hall of Wonders, although these are not necessary for the quest.
It is not possible to get your mage to turn your thief invisible before sending them in, as they will immediately turn visible again once they have taken the telescope.