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June 16, 2003

A visit to Mount Wilson

I'm in Pasadena at the moment (without family). For Father's day, I decided to take a little trip up to the Mt. Wilson observatory.

The San Gabriel Mountains form a northern barrier to expansion of the LA region. The San Gabriels rise steeply just behind Pasadena. Pasadena is ~270 feet altitude; Mt. Wilson is up around 5700' feet.

I made it up for one of the 1pm Saturday/Sunday docent tours. The tour today was hosted by a retired engineer/amateur astronomer who was quite knowledgeable about all the different activities on Mt Wilson.

Mt. Wilson is home to a variety of observatories. I would have expected that the light pollution from the LA basin would have obsoleted Mt. Wilson, but that's not true. Astronomers talk about how good the seeing is at a particular site. Seeing describes the level of detail you can resolve with your telescopes. It's related to how smooth the air is. It surprised me to hear that Mt. Wilson is home to some of the best seeing in all of North America. The same weather patterns that lead to smog in LA keep the air very smooth most of the time at Wilson.

I was surprised about the extent to which observatories are still more like machine shops than computer rooms. I read Timothy Ferris's book Seeing in the Dark recently, which is all about the exploits of amateur astronomers. Ferris' book describes how digital CCDs have taken over from film as the preferred method of capturing images. But at Mt Wilson, maintaining the old 60" and 100" inch telescopes is still a matter of people wielding huge wrenches. On the day I went, they were getting ready to realuminizing the 100" mirror. Every two years or so they have to take the 100" mirror out, and re-coat with a very fine layer of aluminum. Lots of people with some combination of brute strength and fine motor control working with cranes and wrestling with large bolts. (The 100" mirror weighs 9,000 pounds, so you don't "drop" it out of the telescope without a lot of preparation.)

A few factoids:

  • The largest observatories on Mt. Wilson are the 60" and the 100" telescopes, each of which were the largest in the world when they were built. The 60" is now used mostly for amateur viewing, but the 100" has been outfitted with adaptive optics which allow it to rival the Hubble Space Telescope at some types of viewing.
  • Light pollution is a problem, but not for viewing the brightest objects in the sky. Since many observations now use CCDs rather than film, certain techniques even allow astronomers top filter out the light interference from LA. (Imagine taking an unfocused pictures of the sky; this gives you a template for the light pollution coming in that you can subtract out from astronomical observations.) And on occasion LA marine layer keeps all that light under a blanket of clouds while leaving Mt. Wilson in the clear.
  • Mt. Wilson is home to two solar observatories. The larger of the two - the 150' tower - has a unique construction method where one tower is essentially built entirely inside another tower to minimize vibration.
  • Wilson is also home to a Georgia State project called the Chara Array. Chara is a interferometric array; that is, an array of six 40" telescopes that can be combined to resolve very fine details. The principal involved is to combine the light from two separate telescopes, and see where the light from the images interfere with each other. Chara can combine light from telescopes that are as far as 1000' apart, which in a certain sense is like using a telescope with a 1000' mirror. Chara can resolve objects with great detail - for example, being able to tell that something that looks like one object is actually two stars orbiting around each other.