In September 2018, Mum came to visit, and among our adventures (including to Hawai’i) we spent a Saturday afternoon at Mount Wilson Observatory, in the mountains behind Los Angeles.
Mount Wilson is a historic center of astronomy and the place where astronomers discovered that the universe was bigger than just our Milky Way galaxy. The observatory was founded in 1904 and it still has some working telescopes today.
We drove up the Angeles Crest Highway, winding through the mountains, and arrived at the Cosmic Cafe where we bought our docent-led tour tickets ($15 each) and met our guide, Bob. There were about 15 people on the tour, which began at 1pm.
150ft Solar Tower
Our first stop was the 150ft solar tower – one of two solar towers that are just visible from Pasadena. The solar tower was used, you guessed it, to make observations of the sun. It has a lens and a mirror at the top of the tower which projects the light of the sun into a scientific instrument called a spectrograph at its base.
Bob took us into the base of the tower to meet a staff member who had been with Mt Wilson for over 40 years. He showed us an image of the sun projected onto a screen – and a tiny sunspot was visible. He showed us a comparison with the largest ever sunspot from 1947, compared with the size of Jupiter and the Earth. Inside the control room, much of the original equipment was still in place.
100-inch Telescope
Our next stop was the telescope that was once the world’s largest: the 100-inch Hooker telescope. “100-inch” refers to the diameter of the primary mirror that collects the light to allow astronomers to do their science (the biggest telescopes today have mirrors 10 meters in diameter – close to 400 inches). Bob showed us the underside of the 100-inch mirror blank: its production was difficult and the final glass disk is actually full of bubbles due to flaws in the process used to cast it in 1908.
We explored all parts of the telescope and dome, including seeing a drawer of blueprints for the telescope and going under the main floor to see the telescope pier (the concrete structure that holds up the telescope) which also seemed to be a storage area for huge miscellaneous pieces of metal.
The best bit for me though was when we got to go outside on the catwalk of the dome to see some great views.
60-inch Telescope
By this point, the 2-hour tour had been going 2.5 hours but there was more to see and no one wanted to leave. We headed over to the 60-inch telescope, also once the world’s largest telescope, with a stop along the way to a spectacular viewpoint.
Inside the 60-inch, another guide, Tom was getting ready for a public observing night. Tom slewed the telescope for us so we could take a look at the mirror – it was a much clearer piece of glass than the 100-inch.
Our tour finished after 3hr 15 mins. No one was complaining!
More to see
Going to Mount Wilson means climbing to 5700 ft, and naturally the views of the city are spectacular along the way.
This was probably my third or fourth visit to Mount Wilson, having been lucky enough to come on private tours and an evening observing session as well. I highly recommend a visit, or even getting a group together for an evening session.
Perpetually Expat 
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