As you drive into the Angeles National Forest take the Red Box Rd turn-off, which, 5 miles later, dead ends atop 5715ft Mt Wilson. Operating since 1904, this was the world’s top astronomical research facility in the early 20th century and is still in use today.
You can walk around the grounds (download a handy self-guided tour from the website) and visit the museum. Free guided tours run at 1pm on Saturdays and Sundays April to November. The website also has details on how to schedule a viewing session using the 1908 60-inch telescope (half-/full night $1000/1700 for groups of up to 25 people). It’s closed in bad weather.