The ancient astroscope is a relic that may be received from breaking open barronite deposits. A relic from the Second Age, it belonged to the inhabitants of Lassar, who were destroyed by Saradominist armies after the events of the Betrayal of Zaros, which included the Imcando dwarves.
The astroscope can be handed to Curator Haig Halen at the Varrock Museum for 1 Kudos and 200 coins. Players will receive 100 coins for additional ancient astroscopes. When given to the curator for the first time, he will comment "An Ancient astroscope. I have only read of such things but to see one, incredible!"
For an exhaustive list of all known sources for this item, see here