th century Central Tibeten thanka of Guhyasamaja Akshobhyavajra Rubin Museum of Art. assessments Tibetan thanka en th century Central Tibeten thanka of Guhyasamaja Akshobhyavajra Rubin Museum of Art Institution Rubin Museum of Art century Creator Unknown PDold Thangka from Tibet thcentury paintings in the United States Tibetan