Complex sentence: Upon seeing Carina's diary, he realizes she is his daughter, however he chooses not to tell her of her true parentage in order to allow her to keep her idealized picture of her father, whom she imagines to be an astronomer.
Split into simpler sentences: Upon seeing Carina's diary, he realizes she is his daughter. However, he chooses not to tell her of her true parentage in order to allow her to keep her idealized picture of her father, whom she imagines to be an astronomer.